Improve doc strings for some conflict resolution functions.

* subversion/svn/cl.h
  (svn_cl__get_conflict_func_postpone_baton,
   svn_cl__conflict_func_postpone,
   svn_cl__resolve_postponed_conflicts): Better doc strings.

* subversion/svn/conflict-callbacks.c
  (svn_cl__conflict_func_postpone): Remove doc string from here since it is
    in the header file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1438658 13f79535-47bb-0310-9956-ffa450edef68