fix: doc: Document built in check-names exclusions

`check-names` rules ordinarily forbid hostnames to contain any character other than letters, digits and hyphen. There are exceptions for A and AAAA records with an Active Directory Forest prefix (`gc._msdcs`), and for RFC7208 exclusion lookup names (interior labels `_spf`, `_spf_verify` and `_spf_rate`). These exceptions were not previously documented, but now are.

Merge branch 'marka-update-check-names-documentation' into 'main'

See merge request isc-projects/bind9!12554