Clarified comment on arg constraints for ap_get_brigade()

Submitted by:	Joe Schaefer <joe+gmane@sunstarsys.com>
Reviewed by:	Brian Pane


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@100249 13f79535-47bb-0310-9956-ffa450edef68