Docs: REQUIRED in ident_regex, proxy_auth_regex, ext_user_regex

[rousskov]

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.