Document the behavior of the "make discard" feature within `check_access`

[jathanism]

The magic comment acl: make discard can be put in to an ACL term to modify the behavior of the term for the purpose of access checks.

This feature needs to be documented properly, with examples, of course. The impetus behind the feature, too.

See: acl.tools.check_access See: acl.parser.Term.makediscard See: bin/check_access and how it displays terms modified by the acl: make discard comment.