spec: clarify default "true" condition/tag in for/switch statements
spec: clarify default "true" condition/tag in for/switch statements

An absent condition/tag in for and switch statements is equivalent
to the predeclared constant true; not simply the expression true
(which might lead to a locally defined true).

Not a language change.

Fixes issue 7404.

LGTM=iant, r
R=r, iant, rsc, ken
CC=golang-codereviews
https://codereview.appspot.com/68150046

wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old