Add minor 'Warning' directive as defined in current mod_macro.

Add minor 'Warning' directive as defined in current mod_macro.

* server/core.c: add 'Warning' directive by extending the 'Error'
  directive implementation. The 'Error' behavior is slightly changed
  so as to use verbose ap_log_error instead of returning the message.
* docs/manual/mod/core.xml: add documentation for 'Warning'.
* server/config.c: add comment about syntax vs configuration errors.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1418677 13f79535-47bb-0310-9956-ffa450edef68