gitlab-ci: Document that ci-templates refs must be in sync

Otherwise there can be weird breakage.

(Removing the include from .gitlab-ci/lava-gitlab-ci.yml doesn't seem
possible unfortunately:
https://gitlab.freedesktop.org/daenzer/mesa/pipelines/79458)

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>

.gitlab-ci.yml

@@ -29,6 +29,7 @@ variables:
 
 include:
   - project: 'wayland/ci-templates'
+    # Must be the same as in .gitlab-ci/lava-gitlab-ci.yml
     ref: 1f7f57c64ff4ebbf7292e3b7a13600518b8cb24c
     file: '/templates/debian.yml'
 

.gitlab-ci/lava-gitlab-ci.yml

@@ -4,6 +4,7 @@ variables:
 
 include:
   - project: 'wayland/ci-templates'
+    # Must be the same as in .gitlab-ci.yml
     ref: 1f7f57c64ff4ebbf7292e3b7a13600518b8cb24c
     file: '/templates/debian.yml'