Merge branch 'kh/doc-refs-migrate-limitations' into seen

The known limitations of the ref format migration in git refs have
been moved to be displayed as a caution admonition directly under
the description of the migrate subcommand, improving visibility. A
reference to git-maintenance has also been corrected to use the
linkgit macro.

* kh/doc-refs-migrate-limitations:
  doc: refs: linkgit to git-maintenance(1)
  doc: refs: put ref migration warning under the command