doc: update Go 1.13 release notes using relnote

The additions were generated using golang.org/x/build/cmd/relnote.

Change-Id: Ie7322f7d01a2dd4a7bca89b9ef9c1ce93bc2671a
Reviewed-on: https://go-review.googlesource.com/c/go/+/180778Reviewed-by: Bryan C. Mills <bcmills@google.com>