Improve the documentation of 'A +' item state in 'svn help status'.

* subversion/svn/svn.c
  (svn_cl__cmd_table): Explicitly state that 'A +' items are "copies",
   instead of describing them only as "addition with history" (these
   terms are interchangeable but add-with-history might not be intuitive
   to novice users).

Found by: drecute on #svn IRC


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1624530 13f79535-47bb-0310-9956-ffa450edef68