documentation on HOT

Hi,

While working on the 2020-02-10 release announcement, in order to better
describe one of the fixes, I tried to find a reference in the docs that
described what HOT is, how it works, etc. in a user-friendly manner.

This lead me to the acronyms page[1], which lead me to a document in the
repo[2]. The closest thing I found was a blog post that Laurenz Albe
wrote on the topic[3].

Given the importance of HOT, it seems like this would be a good topic to
document. I would suggest something higher-level for general users in
the "Indexes"[4] section, and something lower-level in internals[5]
(which could perhaps be derived from [2]).

Thoughts on this?

Thanks,

Jonathan

[1] https://www.postgresql.org/docs/current/acronyms.html
[2]
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/heap/README.HOT;hb=HEAD
[3]
https://www.cybertec-postgresql.com/en/hot-updates-in-postgresql-for-better-performance/
[4] https://www.postgresql.org/docs/current/indexes.html
[5] https://www.postgresql.org/docs/current/internals.html