os/user: document Current improvements

Update #21867.
Update #68312.
Update #68647.

Change-Id: Ic41d6747c5a54ba28c1292258aa4d318ccb9fe40
Reviewed-on: https://go-review.googlesource.com/c/go/+/604395


Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>