Make doc string consistent with implementation.
Make doc string consistent with implementation.

* subversion/include/private/svn_client_private.h
  (svn_client__get_copy_source): Note that when there's no data to
   return that we set *COPYFROM_REV to SVN_INVALID_REVNUM.

Found by: pburba

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