Document and future-proof the dependency between apr_read_type_e and

ap_input_mode_t.  It's now safe(r) to cast from ap_read_type_e to
ap_input_mode_t.

Submitted by:	Justin Erenkrantz, Sander Striker


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89671 13f79535-47bb-0310-9956-ffa450edef68