spec: minor tweaks
spec: minor tweaks

- more idiomatic examples of pointer types
- show use of _ in examples of function types
- remove "legal:" qualification in examples
  for consistency

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5711054

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