Improve doc strings.

* subversion/libsvn_wc/merge.c
  (merge_text_file): Document the results better.
  (merge_binary_file): Fix an inverted sense in a to-do note.
  (svn_wc__internal_merge): Remove an unhelpful comment.

* subversion/libsvn_wc/wc.h
  (svn_wc__internal_merge): State that this does not change working file
    directly on disk.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1432216 13f79535-47bb-0310-9956-ffa450edef68