pgsql: Tweak some comments related to variable-numbered stats in pgstat

Tweak some comments related to variable-numbered stats in pgstat.c

These comments referred to database objects, but depending on the stats
kind dealt with this may not be true.

Issues found while reviewing a different patch in this area.

Discussion: https://postgr.es/m/ZtXR+CtkEVVE/LHF@ip-10-97-1-34.eu-west-3.compute.internal

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fee2b3ea2ecd0da0c88832b37ac0d9f6b3bfb9a9

Modified Files
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src/backend/utils/activity/pgstat.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)