Document the side effect of discover_changed_paths with svn_client_mergeinfo_log2()

* subversion/include/svn_client.h
  (svn_client_mergeinfo_log2): The revisions provided to the receiver is altered
    by the discover_changed_paths argument.  As opposed to svn_client_log5()
    where it just changes how much info about each revision is provided.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1547889 13f79535-47bb-0310-9956-ffa450edef68