Documentation update for multiple path lock/unlock.

* subversion/include/svn_fs.h
  (struct svn_fs_lock_target_t, struct svn_fs_lock_result_t): Add @since.

* subversion/include/svn_repos.h
  (svn_repos_fs_unlock2, svn_repos_fs_lock): Adjust documentation.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1581956 13f79535-47bb-0310-9956-ffa450edef68