The locking restrictions that apply to svn_repos_recover also
apply to svn_repos_upgrade and svn_repos_freeze.

* subversion/include/svn_repos.h
  (svn_repos_upgrade2, svn_repos_freeze): Add note.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1464243 13f79535-47bb-0310-9956-ffa450edef68