Fix incorrect comment regarding command completion tags

Fix incorrect comment regarding command completion tags

The comment talked about some Asserts which did not exist and also a
variable name which seems to have long since disappeared.

Rewrite the comment in a way that will hopefully stand the test of
time and inform people why we always write "INSERT 0 <nrows>" instead of
"INSERT <nrows>" in the command completion tag for INSERT.

Reviewed-by: Mark Dilger
Discussion: https://postgr.es/m/CAApHDvpiUg09AvvGAVopNAKemA9z-kCmt7Fi6HKauc32bKzx4w@mail.gmail.com