Add GIT_FETCH_EXTRA_FLAGS docs

What does this MR do?

Adds documentation for GIT_EXTRA_FETCH_FLAGS, related to gitlab-runner!2000 (merged) and gitlab-runner#4142 (closed)

Screenshots

I'm not sure does this apply.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry no changelog as docs-only change
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines N/A
• Style guides
• Database guides N/A
• Separation of EE specific content N/A

Availability and Testing

No cross-browser testing needed. Just docs change.

Security

No potential security issues. Just docs change.

documentation docsfeature ~"ci variables"