py/mpconfig.h: Add note that using computed gotos in VM is not C99.

py/mpconfig.h

@@ -383,6 +383,8 @@
 
 // Whether to use computed gotos in the VM, or a switch
 // Computed gotos are roughly 10% faster, and increase VM code size by a little
+// Note: enabling this will use the gcc-specific extensions of ranged designated
+// initialisers and addresses of labels, which are not part of the C99 standard.
 #ifndef MICROPY_OPT_COMPUTED_GOTO
 #define MICROPY_OPT_COMPUTED_GOTO (0)
 #endif