all: be more idiomatic when documenting boolean return values.

Phrases like "returns whether or not the image is opaque" could be
describing what the function does (it always returns, regardless of
the opacity) or what it returns (a boolean indicating the opacity).
Even when the "or not" is missing, the phrasing is bizarre.

Go with "reports whether", which is still clunky but at least makes
it clear we're talking about the return value.

These were edited by hand. A few were cleaned up in other ways.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/11699043