Doc: improve and centralize the documentation for OID alias types.

Doc: improve and centralize the documentation for OID alias types.

Previously, a lot of information about type regclass existed only
in the discussion of the sequence functions.  Maybe that made sense
in the beginning, because I think originally those were the only
functions taking regclass.  But it doesn't make sense anymore.
Move that material to the "Object Identifier Types" section in
datatype.sgml, generalize it to talk about the other reg* types
as well, and add more examples.

Per bug #16991 from Federico Caselli.

Discussion: https://postgr.es/m/16991-bcaeaafa17e0a723@postgresql.org