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?
I'm doubting there is any disagreement that this is needed. Three of us said as much recently [1] while discussing a user question regarding our documentation for expression indexes.
I presently haven't felt moved to fill the need myself as so I was fine with at least providing a high-level summary in the glossary which we also the general user to understand the idea without having to dive into the README. The details for internals can continue to be handled there in the interest of at least getting something committed for the majority of users.