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: Iad7b564dd7e6cbcbd0d216c2530802e086ec49cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/235757
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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