doc: document WER and SEH changes in Go 1.21

doc: document WER and SEH changes in Go 1.21

While here, I've removed the CL 472195 TODO, which I marked as
RELNOTE=yes by mistake.

For #57441
For #57302

Change-Id: I7563140bf307f8d732f0154d02b8fa0735527323
Reviewed-on: https://go-review.googlesource.com/c/go/+/499515
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>