Does PTR pointing to domain is forbidden? If so - which RFC states that. The same with A record pointing at the domain. As I see, usually, people are trying to avoid such configurations. But I would like to be precise here. Is it forbidden or not. Thanks.
Example: funnydomain.com. 3600 IN A 184.108.40.206
220.127.116.11.in-addr.arpa. 7200 IN PTR funnydomain.com.
Your example is completely valid. There is no problem with that at all - it is a very very common configuration.
The main thing you should not do on a domain, but can do on a hostname, is setting a CNAME. This because on the domain name, there will always be NS records and those can never be combined with a CNAME.