Organize the contents of svn_repos.h for clarity, especially with regard to grouping
of declarations for the Doxygen documentation. Update some doc strings.

(A cherry-pick merge of r1607334 from the move-tracking-2 branch where I
accidentally committed it first.)

* subversion/include/svn_repos.h
  (svn_repos_load_uuid): Move into the 'svn_repos_dump_load' group.
  (svn_repos_authz_func_t): Tweak doc string, as it's now widely used.
  (svn_repos_file_rev_handler_t): Move adjacent to where it is used.
  (svn_repos_remember_client_capabilities): Move adjacent to the repository
    capabilities group.
  (svn_repos_fs_change_node_prop): Update documentation about what exactly
    is validated.
  (svn_repos_fs_get_inherited_props): Move into the properties group.
  Add new Doxygen groupings around existing groups of declarations:
  'svn_repos_authz_callbacks', 'svn_repos_notifications',
  'svn_repos_capabilities', 'svn_repos_properties'.
  In the 'svn_repos_dump_load' group, mention verifying as well.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1607346 13f79535-47bb-0310-9956-ffa450edef68