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

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