Reword session docs on prioritizing "cache" vs "cached_db"

[Adam Johnson]

The "using cached sessions" section of the sessions docs (​https://docs.djangoproject.com/en/dev/topics/http/sessions/#using-cached-sessions) lists "cache" first and doesn't really emphasize the downside of its non-persistent sessions. I think "cached_db" should be recommended first, and "cache" listed only as an option when you have a persistent database server.

Users getting logged out "randomly" due to a full/ephemeral cache server is not a good user experience. It also leads to operational complexity. The example that made me check this was ​a forum question from Tim Schilling about migrating between cache servers with the "cache" backend, which is not easy.

[Carlton Gibson]

OK, thanks Adam, happy to look at a re-phrasing.

I think there's some value in mentioning that Redis is persistent under standard configurations, so as not to mislead, whilst we're here.

[Adam Johnson]

Agreed, although it depends on configuration. We could at least mention "only some cache servers and configurations are persistent", or something like that.

[Joseph V Zammit]

PR is open here: ​https://github.com/django/django/pull/16095

[GitHub <noreply@…>]

In fa9ac16c:

Fixed #33797 -- Prioritized cached database backend for cached sessions in docs.

Co-authored-by: Adam Johnson <me@…>

[Mariusz Felisiak <felisiak.mariusz@…>]

In d959d66f:

[4.1.x] Fixed #33797 -- Prioritized cached database backend for cached sessions in docs.

Co-authored-by: Adam Johnson <me@…>
Backport of fa9ac16c1345a7fb6ad500c84961a954529ba2b9 from main