docs: hook up the new USER_NAMES document everywhere

docs: hook up the new USER_NAMES document everywhere

(Also correct the set of names we accept in User=, which was forgotten
to be updated in ae480f0b09aec815b64579bb1828ea935d8ee236.

(cherry picked from commit 887a8fa341d9b24a7c9cd3f1fce328f8e43a1b4f)

Gbp-Pq: Name docs-hook-up-the-new-USER_NAMES-document-everywhere.patch