Added comment explaining 'assert_numbits' error.

lib/nimbase.h

@@ -373,5 +373,8 @@ static inline void GCGuard (void *ptr) { asm volatile ("" :: "X" (ptr)); }
 #  define GC_GUARD
 #endif
 
+/* Test to see if nimrod and the C compiler agrees on the size of a pointer.
+   On disagreement, your C compiler will say something like: 
+   "error: 'assert_numbits' declared as an array with a negative size" */
 typedef int assert_numbits[sizeof(NI) == sizeof(void*) && NIM_INTBITS == sizeof(NI)*8 ? 1 : -1];
 #endif