Parts of the documentation are very good.

For example https://www.postgresql.org/docs/13/wal-configuration.html is done properly: it describes what that part of the program actually does, with the settings that affect it introduced at the appropriate point, separately from the reference documentation which lists the same settings in alphabetical order.

But not all aspects of Postgres get this quality of documentation. It would be good to see a similar treatment of the checks the planner or executor performs to decide whether to permit access to a particular table and column and row, rather than having to deduce it from the separate descriptions of the security features.

(For example, the basic fact that a VIEW accesses a table using the privileges of the view's owner, not those of the user executing the query, is very hard to find in the documentation.)

It is great that you can switch on any page in the documentation to the documentation of another release of PostgreSQL. That is extremely helpful if you are working with different versions.

Not just about postgres internals, but generally about SQL in my case.

Interestingly enough the competition for "biggest impact on my SQL Knowledge"... Is a book about MS Access 97 which taught me enough about how SQL actually works that I could understand Postgres docs.

Same here. I feel like I built several years of my career on reading that documentation and knowing where to find things when other folks didn't, so they'd come and ask me and I'd just send them to the right place.