doc/go_mem.html: don't be clever

doc/go_mem.html: don't be clever

Add a short introductory section saying what most Go
programmers really need to know, which is that you
shouldn't have to read this document to understand
the behavior of your program.

LGTM=bradfitz, adg, tracey.brendan, iant, rsc, dsymonds
R=golang-codereviews, bradfitz, tracey.brendan, adg, iant, rsc, dsymonds
CC=golang-codereviews
https://golang.org/cl/158500043