wikiheaders: Bind #defines below a function to it, like typedefs.

This effectively adds the property symbols for various functions to the
function's wiki page.

include/SDL3/SDL_iostream.h

@@ -375,6 +375,7 @@ extern DECLSPEC int SDLCALL SDL_CloseIO(SDL_IOStream *context);
  */
 extern DECLSPEC SDL_PropertiesID SDLCALL SDL_GetIOProperties(SDL_IOStream *context);
 
+/* Possible `whence` values for SDL_IOStream seeking... */
 #define SDL_IO_SEEK_SET 0       /**< Seek from the beginning of data */
 #define SDL_IO_SEEK_CUR 1       /**< Seek relative to current read point */
 #define SDL_IO_SEEK_END 2       /**< Seek relative to the end of data */