Document the two meanings of 'ignore_ancestry' in libsvn_client merge code.

* subversion/include/svn_client.h
  (svn_client_merge4): Document both meanings. (This doc string also applies,
    by reference, to svn_client_merge_peg4().)

* subversion/libsvn_client/merge.c
  (merge_cmd_baton_t): Document both meanings, briefly.
  (do_file_merge): Update a comment.
  (do_merge): Document both meanings, more fully.
  (merge_locked, merge_peg_locked): Refer to the doc string of do_merge().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1436636 13f79535-47bb-0310-9956-ffa450edef68