doc: document the machine-independent changes to the assembler

The architecture-specific details will be updated and expanded in
a subsequent CL (or series thereof).

Update #10096

Change-Id: I59c6be1fcc123fe8626ce2130e6ffe71152c87af
Reviewed-on: https://go-review.googlesource.com/11954Reviewed-by: Russ Cox <rsc@golang.org>