bug#50416: 27.2; Clarify gnus-group-use-permanent-levels behavior

[Allen Li]

The effect of gnus-group-use-permanent-levels persists across separate
Gnus sessions.

For example, if I start Gnus, set the level to N, quit Gnus, and start
Gnus again, the level I previously set still applies.

I think this behavior is a little surprising and should be explicitly
noted in the documentation.

In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.27, cairo 
version 1.17.4)
 of 2021-03-26 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: Arch Linux