Followup to r1507382: Document that the config_dir paramete can be passed as
NULL and that this means using the default config dir location.

* subversion/include/svn_config.h
  (svn_config_walk_auth_data): Document that config_dir can be NULL


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509186 13f79535-47bb-0310-9956-ffa450edef68