Doc: consistently identify OID catalog columns that can be zero.
Not all OID-reference columns that can contain zeroes (indicating
"no reference") were explicitly marked in catalogs.sgml. Fix that,
and try to make the style a little more consistent while at it ---
for example, consistently write "zero" not "0" for these cases.
Joel Jacobson and Tom Lane
Discussion: https://postgr.es/m/
4ed9a372-7bf9-479a-926c-
ae8e774717a8@www.fastmail.com