Document features deployed behind feature flags

GitLab uses Feature Flags to strategically roll out the deployment of its own features. The way we document a feature behind a feature flag depends on its state (enabled or disabled). When the state changes, the developer who made the change must update the documentation accordingly.

Criteria

According to the process of deploying GitLab features behind feature flags:

By default, feature flags should be off.

Feature flags should remain in the codebase for a short period as possible to reduce the need for feature flag accounting.

In order to build a final release and present the feature for self-managed users, the feature flag should be at least defaulted to on.

See how to document them below, according to the state of the flag:

Features disabled by default.
Features that became enabled by default.
Features directly enabled by default.
Features with the feature flag removed.

NOTE: Note: The **(CORE ONLY)** badge or equivalent for the feature's tier should be added to the line and heading that refers to enabling/disabling feature flags as Admin access is required to do so, therefore, it indicates that it cannot be done by regular users of GitLab.com.

Features disabled by default

For features disabled by default, if they cannot be used yet due to lack of completeness, or if they're still under internal evaluation (for example, for performance implications) do not document them: add (or merge) the docs only when the feature is safe and ready to use and test by end users.

For feature flags disabled by default, if they can be used by end users:

Say that it's disabled by default.
Say whether it's enabled on GitLab.com.
Say whether it can be enabled or disabled per-project.
Say whether it's recommended for production use.
Document how to enable and disable it.

For example, for a feature disabled by default, disabled on GitLab.com, can be enabled or disabled per-project, and not ready for production use:

# Feature Name

> - [Introduced](link-to-issue) in GitLab 12.0.
> - It's deployed behind a feature flag, disabled by default.
> - It's disabled on GitLab.com.
> - It's able to be enabled or disabled per-project
> - It's not recommended for production use.
> - To use it in GitLab self-managed instances, ask a GitLab administrator to [enable it](#anchor-to-section). **(CORE ONLY)**

(...)

### Enable or disable <Feature Name> **(CORE ONLY)**

<Feature Name> is under development and not ready for production use. It is
deployed behind a feature flag that is **disabled by default**.
[GitLab administrators with access to the GitLab Rails console](../path/to/administration/feature_flags.md)
can enable it for your instance. <Feature Name> can be enabled or disabled per-project

To enable it:

```ruby
# Instance-wide
Feature.enable(:<feature flag>)
# or by project
Feature.enable(:<feature flag>, Project.find(<project id>))
```

To disable it:

```ruby
# Instance-wide
Feature.disable(:<feature flag>)
# or by project
Feature.disable(:<feature flag>, Project.find(<project id>))
```

Adjust the blurb according to the state of the feature you're documenting.

Features that became enabled by default

For features that became enabled by default:

Say that it became enabled by default.
Say whether it's enabled on GitLab.com.
Say whether it can be enabled or disabled per-project.
Say whether it's recommended for production use.
Document how to disable and enable it.

For example, for a feature initially deployed disabled by default, that became enabled by default, that is enabled on GitLab.com, that cannot be enabled or disabled per-project, and ready for production use:

# Feature Name

> - [Introduced](link-to-issue) in GitLab 12.0.
> - It was deployed behind a feature flag, disabled by default.
> - [Became enabled by default](link-to-issue) on GitLab 12.1.
> - It's enabled on GitLab.com.
> - It's not able to be enabled or disabled per-project
> - It's recommended for production use.
> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#anchor-to-section). **(CORE ONLY)**

(...)

### Enable or disable <Feature Name> **(CORE ONLY)**

<Feature Name> is under development but ready for production use.
It is deployed behind a feature flag that is **enabled by default**.
[GitLab administrators with access to the GitLab Rails console](..path/to/administration/feature_flags.md)
can opt to disable it for your instance it cannot be enabled or disabled per-project.

To disable it:

```ruby
Feature.disable(:<feature flag>)
```

To enable it:

```ruby
Feature.enable(:<feature flag>)
```

Adjust the blurb according to the state of the feature you're documenting.

Features directly enabled by default

For features enabled by default:

Say it's enabled by default.
Say whether it's enabled on GitLab.com.
Say whether it can be enabled or disabled per-project.
Say whether it's recommended for production use.
Document how to disable and enable it.

For example, for a feature enabled by default, enabled on GitLab.com, cannot be enabled or disabled per-project, and ready for production use:

# Feature Name

> - [Introduced](link-to-issue) in GitLab 12.0.
> - It's deployed behind a feature flag, enabled by default.
> - It's enabled on GitLab.com.
> - It's not able to be enabled or disabled per-project
> - It's recommended for production use.
> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#anchor-to-section). **(CORE ONLY)**

(...)

### Enable or disable <Feature Name> **(CORE ONLY)**

<Feature Name> is under development but ready for production use.
It is deployed behind a feature flag that is **enabled by default**.
[GitLab administrators with access to the GitLab Rails console](..path/to/administration/feature_flags.md)
can opt to disable it for your instance.

To disable it:

```ruby
Feature.disable(:<feature flag>)
```

To enable it:

```ruby
Feature.enable(:<feature flag>)
```

Adjust the blurb according to the state of the feature you're documenting.

Features with flag removed

Once the feature is ready and the flag has been removed, clean up the documentation. Remove the feature flag mention keeping only a note that mentions the flag in the version history notes:

# Feature Name

> - [Introduced](link-to-issue) in GitLab 12.0.
> - [Feature flag removed](link-to-issue) in GitLab 12.2.

(...)