[release-branch.go1.11] doc: document Go 1.11.1

Updates #27953

Change-Id: I2f1a55e15dc5737a5a06bd894c46b2c4705f338c
Reviewed-on: https://go-review.googlesource.com/138858


Reviewed-by: Filippo Valsorda <filippo@golang.org>
(cherry picked from commit f99fc3a1)
Reviewed-on: https://go-review.googlesource.com/138859


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>