all: use "reports whether" consistently instead of "returns whether"

Follow-up for CL 147037 and after Brad noticed the "returns whether"
pattern during the review of CL 150621.

Go documentation style for boolean funcs is to say:

    // Foo reports whether ...
    func Foo() bool

(rather than "returns whether")

Created with:

    $ perl -i -npe 's/returns whether/reports whether/' $(git grep -l "returns whether" | grep -v vendor)

Change-Id: I15fe9ff99180ad97750cd05a10eceafdb12dc0b4
Reviewed-on: https://go-review.googlesource.com/c/150918
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/signal_unix.go

@@ -773,7 +773,7 @@ func unminitSignals() {
 	}
 }
 
-// blockableSig returns whether sig may be blocked by the signal mask.
+// blockableSig reports whether sig may be blocked by the signal mask.
 // We never want to block the signals marked _SigUnblock;
 // these are the synchronous signals that turn into a Go panic.
 // In a Go program--not a c-archive/c-shared--we never want to block