ci: Rename .ci-run-policy rules to .build-rules.

... and explain what they're doing, compared to the test rules in
test-source-dep.yml.

Unfortunately, we can't really pull them into test-source-dep.yml with
other source deps, because of various '&'-'*' references.

Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17386>

.gitlab-ci.yml

@@ -126,7 +126,7 @@ stages:
 .docs-base:
   extends:
     - .fdo.ci-fairy
-    - .ci-run-policy
+    - .build-rules
   script:
   - apk --no-cache add graphviz doxygen
   - pip3 install sphinx breathe mako sphinx_rtd_theme
@@ -177,8 +177,8 @@ test-docs-mr:
       when: on_success
     # Other cases default to never
 
-# When to automatically run the CI
-.ci-run-policy:
+# When to automatically run the CI for build jobs
+.build-rules:
   rules:
     - !reference [.no_scheduled_pipelines-rules, rules]
     # If any files affecting the pipeline are changed, build/test jobs run
@@ -217,7 +217,7 @@ test-docs-mr:
 
 .container-rules:
   extends:
-    - .ci-run-policy
+    - .build-rules
   rules:
     - !reference [.no_scheduled_pipelines-rules, rules]
     # Run pipeline by default in the main project if any CI pipeline