Docs: REQUIRED in ident_regex, proxy_auth_regex, ext_user_regex (#1818)

Docs: REQUIRED in ident_regex, proxy_auth_regex, ext_user_regex (#1818)

The three ACLs were documented as matching any username when configured
with a parameter spelled "REQUIRED". Neither actually treated that
parameter specially -- all interpreted it as an ordinary regex.

This dangerous documentation bug was introduced in 2000 commit 145cf92
that added ident_regex and proxy_auth_regex support. It was then
duplicated in 2003 commit abb929f that added ext_user_regex support.

This minimal documentation fix does not imply that these ACLs should not
treat REQUIRED values specially. Enabling such special treatment
requires significant code changes, especially if we want to do that well
and without duplicating the corresponding code.