Clarify the help for "svn diff".
Clarify the help for "svn diff".

* subversion/svn/main.c
  (svn_cl__cmd_table): Make diff's summary more accurate, and indicate that
    its mode-1 arguments cannot be of mixed type.

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