Follow-up to r27764, document another restriction on svn:mergeinfo values.
Follow-up to r27764, document another restriction on svn:mergeinfo values.

* subversion/include/svn_mergeinfo.h
  (overview of the mergeinfo prop): For now at least we don't consider
  svn:mergeinfo valid if revision ranges have start revisions >= end
  revisions.  I say "For now" because there is some current consideration
  of implementing explicit reversed range svn:mergeinfo.  Also note that
  we *don't* currently enforce this anywhere, that is coming shortly as
  part of a larger change.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion