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