Note the consequences of allow_mixed_revisions for move.

* subversion/include/private/svn_wc_private.h
  (svn_wc__move2): Tweak documentation.

* subversion/include/svn_client.h
  (svn_client_move6): Tweak documentation.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1455849 13f79535-47bb-0310-9956-ffa450edef68