Doc changes for use of pre-defined project variables in include

Adds doc changes for pre defined project variables in
include section of .gitlab-ci.yml

doc/ci/variables/where_variables_can_be_used.md

@@ -27,6 +27,7 @@ There are two places defined variables can be used. On the:
 | `environment:url`                          | yes              | GitLab                 | The variable expansion is made by the [internal variable expansion mechanism](#gitlab-internal-variable-expansion-mechanism) in GitLab.<br/><br/>Supported are all variables defined for a job (project/group variables, variables from `.gitlab-ci.yml`, variables from triggers, variables from pipeline schedules).<br/><br/>Not supported are variables defined in the GitLab Runner `config.toml` and variables created in the job's `script`. |
 | `environment:name`                         | yes              | GitLab                 | Similar to `environment:url`, but the variables expansion doesn't support the following:<br/><br/>- Variables that are based on the environment's name (`CI_ENVIRONMENT_NAME`, `CI_ENVIRONMENT_SLUG`).<br/>- Any other variables related to environment (currently only `CI_ENVIRONMENT_URL`).<br/>- [Persisted variables](#persisted-variables).                                                                                 |
 | `resource_group`                           | yes              | GitLab                 | Similar to `environment:url`, but the variables expansion doesn't support the following:<br/><br/>- Variables that are based on the environment's name (`CI_ENVIRONMENT_NAME`, `CI_ENVIRONMENT_SLUG`).<br/>- Any other variables related to environment (currently only `CI_ENVIRONMENT_URL`).<br/>- [Persisted variables](#persisted-variables).                                                                                 |
+| `include`                                  | yes              | GitLab                 | The variable expansion is made by the [internal variable expansion mechanism](#gitlab-internal-variable-expansion-mechanism) in GitLab. <br/><br/>Predefined project variables are supported: `GITLAB_FEATURES`, `CI_DEFAULT_BRANCH`, and all variables that start with `CI_PROJECT_` (for example `CI_PROJECT_NAME`).                                                                                                            |
 | `variables`                                | yes              | Runner                 | The variable expansion is made by GitLab Runner's [internal variable expansion mechanism](#gitlab-runner-internal-variable-expansion-mechanism)                                                                                                                                                                                                                                                                                   |
 | `image`                                    | yes              | Runner                 | The variable expansion is made by GitLab Runner's [internal variable expansion mechanism](#gitlab-runner-internal-variable-expansion-mechanism)                                                                                                                                                                                                                                                                                   |
 | `services:[]`                              | yes              | Runner                 | The variable expansion is made by GitLab Runner's [internal variable expansion mechanism](#gitlab-runner-internal-variable-expansion-mechanism)                                                                                                                                                                                                                                                                                   |

doc/ci/yaml/README.md

@@ -363,8 +363,6 @@ use the [`extends` keyword](#extends).
 | [`remote`](#includeremote)      | Include a file from a remote URL. Must be publicly accessible.    |
 | [`template`](#includetemplate)  | Include templates that are provided by GitLab.                    |
 
-The `include` methods do not support [variable expansion](../variables/where_variables_can_be_used.md#variables-usage).
-
 `.gitlab-ci.yml` configuration included by all methods is evaluated at pipeline creation.
 The configuration is a snapshot in time and persisted in the database. Any changes to
 referenced `.gitlab-ci.yml` configuration is not reflected in GitLab until the next pipeline is created.
@@ -379,6 +377,48 @@ NOTE:
 Use merging to customize and override included CI/CD configurations with local
 definitions. Local definitions in `.gitlab-ci.yml` override included definitions.
 
+#### Variables with `include`
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/284883) in GitLab 13.8.
+> - It's [deployed behind a feature flag](../../user/feature_flags.md), disabled by default.
+> - It's disabled on GitLab.com.
+> - It's not recommended for production use.
+> - To use it in GitLab self-managed instances, ask a GitLab administrator to [enable it](#enable-or-disable-includepredefined-project-variables). **(CORE ONLY)**
+
+WARNING:
+This feature might not be available to you. Check the **version history** note above for details.
+
+You can [use some predefined variables in `include` sections](../variables/where_variables_can_be_used.md#gitlab-ciyml-file)
+in your `.gitlab-ci.yml`:
+
+```yaml
+include:
+  project: '$CI_PROJECT_PATH'
+  file: '.compliance-gitlab-ci.yml'
+```
+
+For an example of how you can include these predefined variables, and their impact on CI jobs,
+see the following [CI variable demo](https://youtu.be/4XR8gw3Pkos).
+
+##### Enable or disable include:predefined-project-variables **(CORE ONLY)**
+
+Use of predefined project variables in `include` section of `.gitlab-ci.yml` 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](../../administration/feature_flags.md)
+can enable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:variables_in_include_section_ci)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:variables_in_include_section_ci)
+```
+
 #### `include:local`
 
 `include:local` includes a file from the same repository as `.gitlab-ci.yml`.