wikiheaders: optionally generate a Quick Reference page.

Plus some fixes to SDL's headers this brought up.

include/SDL3/SDL_stdinc.h

@@ -125,7 +125,10 @@ void *alloca(size_t);
 #endif
 
 /**
- * The number of elements in an array.
+ * The number of elements in a static array.
+ *
+ * This will compile but return incorrect results for a pointer to an array;
+ * it has to be an array the compiler knows the size of.
  *
  * This macro looks like it double-evaluates the argument, but it does so
  * inside of `sizeof`, so there are no side-effects here, as expressions do