doc/go1.16: update runtime and compiler sections

This resolves all TODOs for the runtime and compiler and mentions
several other changes.

For #40700.
Fixes #42892.
Fixes #42894.

Change-Id: I18d14cfe572baf679ecf8b0a4e82c4b866da5a04
Reviewed-on: https://go-review.googlesource.com/c/go/+/275176


Trust: Austin Clements <austin@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>