docs: upgrade steps needed for all kinds of repos, fixes #7813

Repos with encryption=none of course can not do real authentication (because there is no secret key material to work with).

But the code always works the same way and such repos also use (and expect) TAMs, so the instructions should be followed in any case.

docs/changes.rst

@@ -33,7 +33,8 @@ Below, if we speak of borg 1.2.6, we mean a borg version >= 1.2.6 **or** a
 borg version that has the relevant security patches for this vulnerability applied
 (could be also an older version in that case).
 
-Steps you must take to upgrade a repository:
+Steps you must take to upgrade a repository (this applies to all kinds of repos
+no matter what encryption mode they use, including "none"):
 
 1. Upgrade all clients using this repository to borg 1.2.6.
    Note: it is not required to upgrade a server, except if the server-side borg