Revision 1665437

Correct and complete docstrings in the FS API after 1.9 review.
No functional change.

* subversion/include/svn_fs.h
  (svn_fs_backend_names): New Doxygen group allowing us to refer to the
                          pre-defined set of backends.  Move to front due
                          to issues with group nesting.
  (SVN_FS_CONFIG_COMPATIBLE_VERSION): Works for the current release as well.
  (svn_fs_open2): Correct cleanup behavior description for RESULT_POOL.
                  Document SCRATCH_POOL.
  (svn_fs_upgrade_notify_t): Grammar fix.
  (svn_fs_upgrade2): Refer to latest repos API.
  (svn_fs_hotcopy3): BDB is the only backend not sending notifications.
                     Document CANCEL_FUNC.
  (svn_fs_compare_ids,
   svn_fs_check_related): "Now" is 1.9.
  (svn_fs_props_changed): Fix confusing wording.
  (svn_fs_info_format,
   svn_fs_lock_target_t): For clarity, remove '()' from function names in
                          @see line.
  (svn_fs_fsfs_info_t): Make comment visible to doxygen.
  (svn_fs_info_placeholder_t): For clarity, remove '()' from function names
                               in @see line.
  (svn_fs_info): Mention FSX alongside FSFS.
  (svn_fs_info_dup): Update pool usage documentation.