Document linguist overriding

What does this MR do?

Document a feature that's not known to users, overriding of default linguist behaviour.

Relevant issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/50705

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides