Add some comments explaining what interface a function implements.
No functional changes.

* subversion/libsvn_ra_serf/log.c
  (create_log_body): Add comment.

* subversion/libsvn_ra_serf/merge.c
  (create_merge_body): Add comment.

* subversion/libsvn_ra_serf/mergeinfo.c
  (create_mergeinfo_body): Add comment.

* subversion/libsvn_ra_serf/options.c
  (create_options_body,
   create_simple_options_body): Add comment.

* subversion/libsvn_ra_serf/property.c
  (create_propfind_body): Add comment.

* subversion/libsvn_ra_serf/replay.c
  (create_replay_body): Add comment.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1556799 13f79535-47bb-0310-9956-ffa450edef68