doc/go1.15: update Go 1.15 release notes using relnote

doc/go1.15: update Go 1.15 release notes using relnote

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

Updates #37419

Change-Id: I99e934377e4bb511c44908039ad6d3bfd3ee35ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/241065
Run-TryBot: Carlos Amedee <carlos@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>