Move `%{merge_request_author}` to correct docs page
What does this MR do and why?
Problem reported via slack:
it looks like the %{merge_request_author} template variable is not working. Am I missing something, or is this a bug? I searched for an existing bug report but could not find one.
What: This MR moves %{merge_request_author}
to the correct docs page.
Why: This variable only applies to commit templates, not MR templates.
Source: list of allowed MR template variables: https://gitlab.com/gitlab-org/gitlab/-/blob/426f98a2a89ccdbd295f324218f0838d312184a1/lib/gitlab/merge_requests/message_generator.rb#L102-115
MR that introduced the feature: !152510 (merged)
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