Clarify doc strings related to conflicts.

* subversion/include/svn_wc.h
  (svn_wc_create_conflict_result): Be explicit about the 'save_merged'
    field. Remove wording that may be read as documenting what a future
    implementation will do with any new fields, since we don't need to say
    this until it happens and the way it was described here is not
    necessarily exactly what we would want to do. (All we really promise is
    that it will behave in a backward compatible manner, and that's an
    implicit promise across all of our APIs.)

* subversion/libsvn_wc/conflicts.h
  (svn_wc__conflict_create_markers): Clarify.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1480681 13f79535-47bb-0310-9956-ffa450edef68