Include wording describing the usage of peg revision in the help message for 'svn export'.
Include wording describing the usage of peg revision in the help
message for 'svn export'.

* subversion/clients/cmdline/main.c
  (svn_cl__cmd_table): Reference the possible peg revision that the
    targets of the export command can now take.

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