Clarify when CI_COMMIT_BRANCH is present

What does this MR do?

Clarify when CI_COMMIT_BRANCH is present in the docs.

Fix #238601 (closed)

Discussion: !31127 (comment 445297643)

• Related to #217668 (closed)
• Related to #273209 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
  • "Any docs-only changes should not have a changelog entry."
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content