Make 'svn help diff' explain more clearly how to show local changes.

* subversion/svn/svn.c
  (svn_cl__cmd_table): List 'svn diff' as the first of all possible ways of
   invoking the 'diff' subcommand, rather than mentioning it just briefly in
   the help text body. Renumber all items in the invocation list accordingly.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442719 13f79535-47bb-0310-9956-ffa450edef68