Improve explanation of the XXTEA algorithm

puer.c

@@ -25,16 +25,18 @@ void xxtea128(uint32_t const key[4], uint32_t block[4]) {
 		//    key accesses, since key is only 4 words long
 		//
 		// 3. Go through each word in the block and derive its new
-		//    value based on next (y) and previous word (z),
-		//    wrapping around as needed, as well as the round
-		//    constant(s) and the key.
+		//    value based on its current value (v[p]), the next (y)
+		//    and the previous word (z), wrapping around the ends
+		//    of the block as needed.
 		//
-		// The function for deriving the new value of a block is a
-		// xor of sums of xors. The first sum adds together
-		// combinations of the next and previous block, and the
-		// second sum adds together previous/bext combined with a
-		// value dependant on the round constant. The key is also
-		// mixed into the block in the first xor of second sum.
+		// The function for deriving the new value of a word is a
+		// xor of sums of xors, followed by an in-place addition.
+		// The first sum adds together combinations of the next and
+		// previous word, and the second sum adds together
+		// previous/next combined with a value dependant on the
+		// round constant. The key is also mixed into the word in
+		// the first xor of second sum. After this the result is
+		// added back into the original word.
 		//
 		// I have changed the operand order in the second xor of
 		// first add and in the second add. This is to keep the