[PATCH] Doc: improve documentation of oid columns that can be zero. (correct version)

Please ignore previous email, the attached file was 0 bytes.
Here comes the patch again, now including data.

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Doc: improve documentation of oid columns that can be zero.

pg_attribute.atttypid
Zero if column is dropped.

pg_class.relam
Can be zero, e.g. for views.

pg_depend.classid
Zero for pinned objects.

pg_language.lanplcallfoid
Zero for internal languages.

pg_operator.oprcode
Zero if none.

pg_operator.oprcom
Zero if none.

pg_operator.oprjoin
Zero if none.

pg_operator.oprnegate
Zero if none.

pg_operator.oprrest
Zero if none.

pg_operator.oprresult
Zero if none.

pg_policy.polroles
Array with a zero element if none.

pg_shdepend.classid
Zero for pinned objects (deptype='p'),
meaning there is no dependent object.

pg_shdepend.objid
Zero if none.

pg_trigger.tgconstrindid
Zero if none.

pg_trigger.tgconstrrelid
Zero if none.