docs: Document GitLab custom CI/CD variables

We introduced the QEMU_CI_AVOCADO_TESTING variable in commit f56bf4caf
("gitlab: Run Avocado tests manually (except mainstream CI)"), but
forgot to document it properly. Do it now.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210727142431.1672530-2-philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.gitlab-ci.yml

@@ -16,24 +16,9 @@
 # QEMU CI jobs are based on templates. Some templates provide
 # user-configurable options, modifiable via configuration variables.
 #
-# These variables can be set globally in the user's CI namespace
-# setting:
-# https://docs.gitlab.com/ee/ci/variables/#create-a-custom-variable-in-the-ui
-# or set manually each time a branch/tag is pushed, as a git-push
-# command line argument:
-# https://docs.gitlab.com/ee/user/project/push_options.html#push-options-for-gitlab-cicd
+# See https://qemu-project.gitlab.io/qemu/devel/ci.html#custom-ci-cd-variables
+# for more information.
 #
-# Example setting the QEMU_CI_EXAMPLE_VAR variable:
-#
-#   git push -o ci.variable="QEMU_CI_EXAMPLE_VAR=value" myrepo mybranch
-#
-# ----------------------------------------------------------------------
-#
-# List of environment variables that can be use to modify the set
-# of jobs selected:
-#
-# - QEMU_CI_AVOCADO_TESTING
-#   If set, tests using the Avocado framework will be run
 
 include:
   - local: '/.gitlab-ci.d/qemu-project.yml'