doc patch: note AttributeRelationId passed to FDW validator function

Hi

Here:

https://www.postgresql.org/docs/current/fdw-functions.html

the enumeration of OIDs which might be passed as the FDW validator function's
second argument omits "AttributeRelationId" (as passed when altering
a foreign table's column options).

Attached v1 patch adds this to this list of OIDs.

The alternative v2 patch adds this to this list of OIDs, and also
formats it as an
SGML list, which IMHO is easier to read.

Looks like this has been missing since 9.3.

Regards

Ian Barwick