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>

doc/next/6-stdlib/99-minor/os/user/21867.md

+On Windows, [Current] can now be used in Windows Nano Server.
+The implementation has been updated to avoid using functions
+from the `NetApi32` library, which is not available in Nano Server.

doc/next/6-stdlib/99-minor/os/user/68312.md

+On Windows, [Current] has been made considerably faster when
+the current user is joined to a slow domain, which is the
+usual case for many corporate users. The new implementation
+performance is now in the order of milliseconds, compared to
+the previous implementation which could take several seconds,
+or even minutes, to complete.

doc/next/6-stdlib/99-minor/os/user/68647.md

+On Windows, [Current] now returns the process owner user when
+the current thread is impersonating another user. Previously,
+it returned an error.