fix: doc: document optional statements the same, enabled or not

The automatically-generated grammar for named.conf clauses that may or may not be enabled at compile time will now include the same comment, regardless of whether or not they are. Previously, the grammar didn't include a comment if an option was enabled, but said "not configured" if it was disabled. Now, in both cases, it will say "optional (only available if configured)".

Closes #4960