doc/asm: add a section on go_asm.h, clean up go_tls.h section

Currently the only mention of go_asm.h is buried in a confusing
section about the runtime-specific go_tls.h header. We actually want
people to use go_asm.h, so this CL adds a section with a proper
discussion of this header. As part of this, we remove the discussion
of go_asm.h from the go_tls.h section and clean up what remains.

I stumbled on this when working on the internal ABI specification. I
wanted to refer to stable documentation on how to access struct fields
from assembly and found there was none.

Change-Id: I0d53741e7685e65794611939e76285f7c82e1d65
Reviewed-on: https://go-review.googlesource.com/c/go/+/286052


Trust: Austin Clements <austin@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>