All: Declare sim_stop_messages as an array SCPE_BASE long

This avoids a potential invalid pointer dereference when formatting
the return value from sim_instr() if it is < SCPE_BASE but greater
than the previously defined static array size.sizeof

Update simh.doc to reflect this generic change.

Interdata/id32_sys.c

@@ -90,7 +90,7 @@ DEVICE *sim_devices[] = {
     NULL
     };
 
-const char *sim_stop_messages[] = {
+const char *sim_stop_messages[SCPE_BASE] = {
     "Unknown error",
     "Reserved instruction",
     "HALT instruction",