bug#46775: 27.1; ERC: Track: Modified channels doc and `erc-track-find-face' fixes

[Amin Bandali]

Hi Olivier,

Many thanks for your patches, and my apologies it's taken so long to get
them merged.

Olivier Certner writes:

> Patches slightly edited (commit message, doc text) and rebased on top of a 
> recent 'master'. Ready to apply. Easy to backport to 27 as well.

I applied (1) and (2) to 'emacs-27' and then merged that into 'master',
then applied (3) and (4) on 'master'; each with a few small tweaks.

Thanks again,
amin