Documentation and potential upgrade issues relating to switch of login programs from shadow to util-linux

The shadow package historically provided more functional versions of binaries such as /bin/login than Busybox. The recent MR !28688 (merged) moved shadow's /bin/login from the main shadow package to a shadow-login subpackage.

This means that whenever users who are currently using the shadow versions of those binaries upgrade their systems (whether from 3.x to Edge, or to v3.16 when it is released, or even just upgrade on Edge itself) there is a likelyhood that the upgrade will result in util-linux's login being used instead as the shadow-login and util-linux-login packages both provide the virtual package "login-utils" but util-linux-login is preferred (via "provider_priority").

It does appear that the util-linux-login binaries are built with PAM support (added 1.5 years ago, https://git.alpinelinux.org/aports/commit/main/util-linux/APKBUILD?id=c8e669399eef45338db6c6633baaf157c49eb76e) but it is unclear if the login-related utils behave identically to the equivalent utils (both for PAM and non-PAM functionality) from the shadow-login package.

This may potentially impact on users currently relying on such things as PAM-enabled login (such as 2FA provided via PAM TOTP module) - there is a risk that either logins could be blocked or existing security measures become ineffective if the util-linux-login versions behave differently.

At the very least this change in installation/upgrade behaviour should be documented in the Alpine 3.16 Release Notes when it comes out, especially in the "upgrading" section.