add byte literals to asm syntax design document

ASM-SYNTAX.md

@@ -12,11 +12,18 @@ it does not seem like a trivial task. Sue me.
 
 ```
 asm
-	instructions
+	expressions
 
-instructions
+expressions
+	expression
+	expression '\n' expressions
+
+expression
 	instruction
-	instruction '\n' instructions
+	byte-literal
+
+byte-literal
+	'BYTE' digit(0, 255)
 
 instruction
 	mov