doc: update Go 1.13 release notes using relnote
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/+/180778
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

akutz/go,golang/go,golang/go,CAFxX/go,golang/go,nawawi/go,akutz/go,cloudflare/go,golang/go,akutz/go,cloudflare/go,akutz/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,akutz/go,nawawi/go,nawawi/go,akutz/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go