Add a comment about an assumption we make in our keepalive buffering.

Delay the check for "too many items in an iovec" until we actually try to
put something in there. This allows that N+1 bucket to be an EOS, FLUSH,
FILE, or zero-length bucket without triggering a split. Only if that next
bucket has iovec data will a split be made.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88969 13f79535-47bb-0310-9956-ffa450edef68