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>

common.go

@@ -240,7 +240,7 @@ const (
 	RequireAndVerifyClientCert
 )
 
-// requiresClientCert returns whether the ClientAuthType requires a client
+// requiresClientCert reports whether the ClientAuthType requires a client
 // certificate to be provided.
 func requiresClientCert(c ClientAuthType) bool {
 	switch c {