[release-branch.go1.2] src/cmd/?a: link to new assembler document

««« CL 18430044 / 1a86e8314ff5
src/cmd/?a: link to new assembler document
Blocked on 20930043, the CL the new text references.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/18430044
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/25480046