Explicitly state the limitations of our freeze API.  Since r1619068,
we can't error out on these conditions anymore.  No functional change.

* subversion/include/svn_fs.h
  (svn_fs_freeze): List limitations an their implications.

* subversion/include/svn_repos.h
  (svn_repos_freeze): Same.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619070 13f79535-47bb-0310-9956-ffa450edef68