spec: clarify language about unary operator &
spec: clarify language about unary operator &

A composite literal may be parenthesized when
used as operand for the unary operator &.

R=rsc, iant, r, ken
CC=golang-dev
https://codereview.appspot.com/6996053

dsriram/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,dsriram/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,gnanderson/go,dsriram/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang