document optional statements the same, enabled or not

document optional statements the same, enabled or not

the generated grammar for named.conf clauses that may or may not be
enabled at compile time will now print the same comment regardless of
whether or not they are.

previously, the grammar didn't print a comment if an option was enabled,
but printed "not configured" if it was disabled. now, in both cases,
it will say "optional (only available if configured)".

as an incidental fix, clarified the documentation for "named-checkconf -n".