* subversion/include/svn_client.h   (svn_client_export4):  Clarify that NULL arguments are not allowed.
* subversion/include/svn_client.h
  (svn_client_export4):  Clarify that NULL arguments are not allowed.

Found by: Jeremy Whitlock

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