Incorrect comment regarding command completion tags

I was looking at the code in EndCommand() and noticed a comment
talking about some Asserts which didn't seem to exist in the code.
The comment also talks about LastOid which looks like the name of a
variable that's nowhere to be seen.

It looks like the Asserts did exists when the completion tag patch was
being developed [1] but they disappeared somewhere along the line and
the comment didn't get an update before 2f9661311 went in.

In the attached, I rewrote the comment to remove mention of the
Asserts. I also tried to form the comment in a way that's more
understandable about why we always write a "0" in "INSERT 0 <nrows>".

David

[1] https://www.postgresql.org/message-id/1C642743-8E46-4246-B4A0-C9A638B3E88F@enterprisedb.com