crypto/subtle: normalize constant time ops docs

ConstantTimeCompare is fairly useless if you can't rely on it being zero
when the slices are different, but thankfully it has that property
thanks to the final ConstantTimeByteEq.

Change-Id: Id51100ed7d8237abbbb15778a259065b162a48ad
Reviewed-on: https://go-review.googlesource.com/c/158643Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/subtle/constant_time.go

@@ -6,9 +6,9 @@
 // code but require careful thought to use correctly.
 package subtle
 
-// ConstantTimeCompare returns 1 if and only if the two slices, x
-// and y, have equal contents. The time taken is a function of the length of
-// the slices and is independent of the contents.
+// ConstantTimeCompare returns 1 if the two slices, x and y, have equal contents
+// and 0 otherwise. The time taken is a function of the length of the slices and
+// is independent of the contents.
 func ConstantTimeCompare(x, y []byte) int {
 	if len(x) != len(y) {
 		return 0
@@ -23,7 +23,7 @@ func ConstantTimeCompare(x, y []byte) int {
 	return ConstantTimeByteEq(v, 0)
 }
 
-// ConstantTimeSelect returns x if v is 1 and y if v is 0.
+// ConstantTimeSelect returns x if v == 1 and y if v == 0.
 // Its behavior is undefined if v takes any other value.
 func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }