spec: removed old or invalid TODOs
spec: removed old or invalid TODOs

Several old TODOs are either resolved now (e.g. when is a return
needed), or are from a time the language wasn't frozen (^ for uints
only). Consolidated the others.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9599044

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