Add docs for enabling feature flag by project

- Right now the docs do not mention that a feature flag can potentially
  be enabled/disabled on a project by project basis. This commit adds
that documentation.

doc/administration/feature_flags.md

@@ -94,6 +94,18 @@ Example, to disable Evidence Collection:
 Feature.disable(:release_evidence_collection)
 ```
 
+Some feature flags can be enabled or disabled on a per project basis:
+
+```ruby
+Feature.enable(:<feature flag>, Project.find(<project id>))
+```
+
+For example, to enable the [`:release_evidence_collection`](../ci/junit_test_reports.md#enabling-the-feature) feature flag for project `1234`:
+
+```ruby
+Feature.enable(:release_evidence_collection, Project.find(1234))
+```
+
 When the feature is ready, GitLab will remove the feature flag, the option for
 enabling and disabling it will no longer exist, and the feature will become
 available in all instances.

doc/development/documentation/feature_flags.md

@@ -43,10 +43,11 @@ 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, and
+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:
 
 ````markdown
@@ -55,6 +56,7 @@ not ready for production use:
 > - [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)**
 
@@ -65,18 +67,24 @@ not ready for production use:
 <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.
+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>))
 ```
 ````
 
@@ -88,10 +96,11 @@ 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, and ready for production use:
+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:
 
 ````markdown
 # Feature Name
@@ -100,6 +109,7 @@ For example, for a feature initially deployed disabled by default, that became e
 > - 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)**
 
@@ -110,7 +120,7 @@ For example, for a feature initially deployed disabled by default, that became e
 <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.
+can opt to disable it for your instance it cannot be enabled or disabled per-project.
 
 To disable it:
 
@@ -133,10 +143,11 @@ 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, and ready for production use:
+For example, for a feature enabled by default, enabled on GitLab.com, cannot be enabled or disabled per-project, and ready for production use:
 
 ````markdown
 # Feature Name
@@ -144,6 +155,7 @@ For example, for a feature enabled by default, enabled on GitLab.com, and ready
 > - [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)**