Misleading comment in tuplesort_set_bound

While digging into the incremental sort patch, I noticed in
tuplesort.c at the beginning of the function in $SUBJECT we have this
comment and assertion:

tuplesort_set_bound(Tuplesortstate *state, int64 bound)
{
/* Assert we're called before loading any tuples */
Assert(state->status == TSS_INITIAL);

But AFAICT from reading the code in puttuple_common the state remains
TSS_INITIAL while tuples are inserted (unless we reach a point where
we decide to transition it to TSS_BOUNDED or TSS_BUILDRUNS).

Therefore it's not true that the assertion guards against having
loaded any tuples; rather it guarantees that we remain in standard
memory quicksort mode.

Assuming my understanding is correct, I've attached a small patch to
update the comment to "Assert we're still in memory quicksort mode and
haven't transitioned to heap or tape mode".

Note: this also means the function header comment "Must be called
before inserting any tuples" is a condition that isn't actually
validated, but I think that's fine given it's not a new problem and
even more so since the same comment goes on to say that that's
probably not a strict requirement.

James Coleman