add RFC references for rdata types.
add RFC references for rdata types.

pecharmin/bind9,each/bind9-collab,each/bind9-collab,each/bind9-collab,each/bind9-collab,pecharmin/bind9,pecharmin/bind9,pecharmin/bind9,each/bind9-collab,pecharmin/bind9,pecharmin/bind9,each/bind9-collab