Restrict custom binary and graphics digits (#1693)

* Restrict custom binary and graphics digits

* Update documentation

* Fix build error

test/asm/null-outside-string.asm

@@ -1,7 +1,7 @@
 SECTION "test", ROM0
 	; '\0' is not special here; it's lexed as a line continuation...
-	DEF foo\0bar EQU 42
-	db foo\0bar
+	DEF foo\0qux EQU 42
+	db foo\0qux
 	; ...just like any other non-whitespace character
 	DEF spam\Xeggs EQU 69
 	db spam\Xeggs