[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 f99fc3a119dbb98fa9dddcb2e31a6c51925fde77)
    Reviewed-on: https://go-review.googlesource.com/138859
    Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>