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