Document the pseudo-instruction "bad"

src/mos6502.exec.c

@@ -9,6 +9,12 @@
 #include "mos6502.h"
 #include "mos6502.enums.h"
 
+/*
+ * Log the attempt to execute a "bad" (which is to say, _undefined_)
+ * instruction, and exit the program. If we get to this instruction,
+ * there's probably a bug in the program. (Or you're passing in an
+ * invalid disk image--one of the two.)
+ */
 DEFINE_INST(bad)
 {
     log_critical("Invalid instruction: %2x @ %4x",