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

The known limitations of the ref format migration in 'git refs' have
been moved to be displayed as a warning 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