Add a missing comment for PGPROC.pgprocno

Hi hackers,

I tried to understand some implementation details of ProcArray and
discovered that this is a bit challenging to do due to a missing
comment for PGPROC.pgprocno. E.g. it's hard to understand why
ProcArrayAdd() preserves procArray->pgprocnos[] sorted by (PGPROC *)
if actually the sorting is done by pgprocno. Took me some time to
figure this out.

Here is the patch that fixes this. Hopefully this will save some time
for the newcomers.

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Best regards,
Aleksander Alekseev