windows: update NewLazyDLL, LoadDLL docs to point to NewLazySystemDLL

Point users to the NewLazySystemDLL utility that was added in CL 21592.

Change-Id: I5fddd927fe6628f06a6266b225949c4227fb79f1
GitHub-Last-Rev: 1fe36ed335f040797ae2c778188e40c29604f135
GitHub-Pull-Request: golang/sys#240
Reviewed-on: https://go-review.googlesource.com/c/sys/+/638715
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>