util_script.h: clarify ap_scan_script_header_* behavior

The ap_scan_script_header_* functions were documented to be pass/fail,
but they also have a third "state" that is returned when HTTP conditions
(If-Modified-Since, etc.) aren't met. This has led to some recent bugs,
so document it.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1760404 13f79535-47bb-0310-9956-ffa450edef68