all: use "reports whether" consistently in the few places that didn't

Go documentation style for boolean funcs is to say:

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

(rather than "returns true if")

This CL also replaces 4 uses of "iff" with the same "reports whether"
wording, which doesn't lose any meaning, and will prevent people from
sending typo fixes when they don't realize it's "if and only if". In
the past I think we've had the typo CLs updated to just say "reports
whether". So do them all at once.

(Inspired by the addition of another "returns true if" in CL 146938
in fd_plan9.go)

Created with:

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

Change-Id: Ided502237f5ab0d25cb625dbab12529c361a8b9f
Reviewed-on: https://go-review.googlesource.com/c/147037
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/asm/internal/lex/input.go

@@ -139,7 +139,7 @@ func (in *Input) Text() string {
 	return in.text
 }
 
-// hash processes a # preprocessor directive. It returns true iff it completes.
+// hash processes a # preprocessor directive. It reports whether it completes.
 func (in *Input) hash() bool {
 	// We have a '#'; it must be followed by a known word (define, include, etc.).
 	tok := in.Stack.Next()