Document that variables can be expanded in `tags:`

Commit dfa6e574 "Evaluate the variables for runner tags section"
landed in GitLab 14.1 enabled the expansion of variables in the `tags:`
key so let's add that to the places where expansion happens.
Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

doc/ci/variables/where_variables_can_be_used.md

@@ -28,6 +28,7 @@ There are two places defined variables can be used. On the:
 | `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/>- `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`).                                                                                                            |
+| `tags`                                     | yes              | GitLab                 | The variable expansion is made by the [internal variable expansion mechanism](#gitlab-internal-variable-expansion-mechanism) in GitLab. [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/35742) in GitLab 14.1.                                                                                          |
 | `variables`                                | yes              | GitLab/Runner          | The variable expansion is first made by the [internal variable expansion mechanism](#gitlab-internal-variable-expansion-mechanism) in GitLab, and then any unrecognized or unavailable variables are expanded 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)                                                                                                                                                                                                                                                                                   |