pgsql: Document relaxed HOT for summarizing indexes

Document relaxed HOT for summarizing indexes

Commit 19d8e2308b allowed a weaker check for HOT with summarizing
indexes, but it did not update README.HOT. So do that now.

Patch by Matthias van de Meent, minor changes by me. Backpatch to 16,
where the optimization was introduced.

Author: Matthias van de Meent
Reviewed-by: Tomas Vondra
Backpatch-through: 16
Discussion: https://postgr.es/m/CAEze2WiEOm8V+c9kUeYp2BPhbEc5s473fUf51xNeqvSFGv44Ew@mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1124cb2cf2ebc1b0adb9c5289e53ffe094ce3206

Modified Files
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src/backend/access/heap/README.HOT | 63 +++++++++++++++++++++++++-------------
1 file changed, 42 insertions(+), 21 deletions(-)