spec: shadowed return parameters may be disallowed
spec: shadowed return parameters may be disallowed

This documents the implemented behavior of both
gc and gccgo as an implementation restriction.

NOT A LANGUAGE CHANGE.

Fixes issue 5425.

LGTM=rsc, r, iant
R=r, iant, rsc, ken
CC=golang-codereviews
https://codereview.appspot.com/71430043

tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme