spool_reqbody_cl(): mention in commentary that any data we

read into memory before reaching our threshold will not be
written to disk

tweak an existing comment, since "also" does not apply to
the situation where we chunk due to an environment variable
setting


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/proxy-reqbody@151147 13f79535-47bb-0310-9956-ffa450edef68