Not sure if this page has some incoming links from somewhere, but being on
the page
https://www.postgresql.org/docs/release/13.0/ I naturally replaced the
version with 9.1 (omitting the minor part) -- and found something
unexpected:
https://www.postgresql.org/docs/release/9.1/ -- has "E.99.1. Migration to
Version 9.0.1", which is, obviously, not what is expected.
Moreover, the right sidebar has links to non-existnent versions such as
9.9: https://www.postgresql.org/docs/release/9.9/.
If I entered the full version, with minor part (
https://www.postgresql.org/docs/release/9.2.0/) it would be okay, of course.