doc/go1.2.html: first cut

doc/go1.2.html: first cut
Lay out the doc and write text for the minor changes.
(I left the net ones for someone who understands them better,
or for someone to describe them to me better so I can write them.)
Much still to do.

Delete go1.2.txt so there's only one thing to update.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/13597044