update comments for *_client_block functions.

note namespace protected function names.  clarify where 100 Continue is sent.  remove 
dire warning for calling ap_should_client_block more than once since side effects
appear to be long gone.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@158791 13f79535-47bb-0310-9956-ffa450edef68