Clarify some z/OS mysteries via code comments.

Followup to r1421305.
PR 56210

Backport of r1756163 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x-openssl-1.1.0-compat@1756164 13f79535-47bb-0310-9956-ffa450edef68