doc: Add note about generated columns in foreign tables
doc: Add note about generated columns in foreign tables

Explain that it is not enforced that querying a generated column
returns data that is consistent with the data that was stored.  This
is similar to the note about constraints nearby.

Reported-by: Amit Langote <3ec6203ef41c7efedde580f096e4a791e5bc54ba@gmail.com>

adam8157/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb