util: add some comments

address review comment

store/localstore/engine/engine.go

@@ -49,7 +49,7 @@ type Iterator interface {
 	Value() []byte
 	// Seek moves the iterator to the first key/value pair whose key is greater
 	// or equal to the given key.
-	// It returns whether such pair exist.
+	// It returns whether such pair exists or not.
 	Seek(startKey []byte) bool
 	// Release releases current iterator.
 	Release()

store/localstore/mvcc_test.go

@@ -281,5 +281,10 @@ func (t *testMvccSuite) TestBufferedIterator(c *C) {
 	it, err = tx.Seek([]byte{0xff, 0xff, 0xff, 0xff}, nil)
 	c.Assert(err, IsNil)
 	c.Assert(it.Valid(), IsFalse)
+
+	it, err = tx.Seek([]byte{0x0, 0xff}, nil)
+	c.Assert(err, IsNil)
+	c.Assert(it.Valid(), IsTrue)
+	c.Assert(it.Value(), DeepEquals, []byte("2"))
 	tx.Commit()
 }

util/bytes/bytes.go

@@ -1,6 +1,6 @@
 package bytes
 
-// CloneBytes returns a deep copy of slice b
+// CloneBytes returns a deep copy of slice b.
 func CloneBytes(b []byte) []byte {
 	return append([]byte(nil), b...)
 }

util/bytes/bytes_test.go

@@ -15,6 +15,11 @@ func (s *testBytesHelperSuites) TestBytesClone(c *C) {
 	shadowB := b
 	newB := CloneBytes(b)
 	c.Assert(b, DeepEquals, newB)
+	// Ugly hacks.Go doesn't allow compare two slice (except nil).
+	// For example:  b == newB <-- it's invalid.
+	// In this test, we must ensure CloneBytes method returns a new slice with
+	// the same value, so we need to check the new slice's address.
 	c.Assert(fmt.Sprintf("%p", b) != fmt.Sprintf("%p", newB), IsTrue)
+	// But the addresses are the same when it's a shadow copy.
 	c.Assert(fmt.Sprintf("%p", b), Equals, fmt.Sprintf("%p", shadowB))
 }