[PATCH] Comments related to "Take fewer snapshots" and "Revert patch for taking fewer snapshots"

Hello, hackers.

Yesterday I have noticed that in simple protocol mode snapshot is
taken twice - first time for parsing/analyze and later for execution.

I was thinking it is a great idea to reuse the same snapshot. After
some time (not short) I was able to find this thread from 2011 with
exactly same idea (of course after I already got few % of performance
in POC):

https://www.postgresql.org/message-id/flat/CA%2BTgmoYqKRj9BozjB-%2BtLQgVkSvzPFWBEzRF4PM2xjPOsmFRdw%40mail.gmail.com

And it was even merged: "Take fewer snapshots" (
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=d573e239f03506920938bf0be56c868d9c3416da
)

But where is optimisation in the HEAD?

It absent because was reverted later in 2012 because of tricky reasons
( https://www.postgresql.org/message-id/flat/5075D8DF.6050500%40fuzzy.cz
) in commit "Revert patch for taking fewer snapshots." (
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=532994299e2ff208a58376134fab75f5ae471e41
)

I think it is good idea to add few comments to code related to the
topic in order to safe time for a next guy.

Comments-only patch attached.

Thanks,
Michail.