spec: clarify rules for blank identifiers
spec: clarify rules for blank identifiers

This documents the status quo more precisely.
Not a language change.

Fixes issue 6006.

R=r, rsc, iant, ken
CC=golang-dev
https://codereview.appspot.com/14415043

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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