doc/go1.17: add release notes for register ABI

Also delete the TODO for the linker section.

Updates #44513.
Updates #40724.

Change-Id: I4d62a907e8c3070831a052cdfe1e21648698df12
Reviewed-on: https://go-review.googlesource.com/c/go/+/323289


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>