doc/go1.16: document riscv64 port changes
doc/go1.16: document riscv64 port changes

For #36641
For #40700

Change-Id: Ib268559a2ce7839372dbf273d95876d8d4521a45
Reviewed-on: https://go-review.googlesource.com/c/go/+/274478
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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