Clarify the doxygen documentation for the semantics of the @a sep_chars 
parameter.

* subversion/include/svn_string.h 
  (svn_cstring_split): Clarify doxygen comment.

Suggested by: danielsh
Approved by: danielsh



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1477635 13f79535-47bb-0310-9956-ffa450edef68