doc,examples/sum: improve doc on sum example

Updates #13

examples/sum/asm.go

@@ -12,15 +12,25 @@ func main() {
 	Doc("Sum returns the sum of the elements in xs.")
 	ptr := Load(Param("xs").Base(), GP64())
 	n := Load(Param("xs").Len(), GP64())
+
+	// Initialize sum register to zero.
 	s := GP64()
 	XORQ(s, s)
+
+	// Loop until zero bytes remain.
 	Label("loop")
 	CMPQ(n, Imm(0))
 	JE(LabelRef("done"))
+
+	// Load from pointer and add to running sum.
 	ADDQ(Mem{Base: ptr}, s)
+
+	// Advance pointer, decrement byte count.
 	ADDQ(Imm(8), ptr)
 	DECQ(n)
 	JMP(LabelRef("loop"))
+
+	// Store sum to return value.
 	Label("done")
 	Store(s, ReturnIndex(0))
 	RET()