Improve ExecModifyTable comments.
Some of these comments wrongly implied that only an AFTER ROW trigger
will cause a 'wholerow' attribute to be present for a foreign table,
but a BEFORE ROW trigger can have the same effect. Others implied
that it would always be present for a foreign table, but that's not
true either.
Etsuro Fujita and Robert Haas
Discussion: http://postgr.es/m/
10026bc7-1403-ef85-9e43-
c6100c1cc0e3@lab.ntt.co.jp