Elaborate on the need to re-login on Debian-based systems to use tor following usermod

Summary:

Starting bitcoind with -onlynet=onion immediately after adding bitcoind user to debian-tor group will yield the following notice on debug.log:

"tor: Authentication cookie /run/tor/control.authcookie could not be opened (check permissions)"

Elaborate on the need to re-login to ensure debian-tor group has been applied to bitcoind user after:

sudo usermod -a -G debian-tor username

Verification can be done via groups command in shell.

Backport of Core PR16857
Depends of D8002

Test Plan: Proof-reading

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8003