wikiheaders: bridge wiki Category docs to the headers!

Did an initial cleanup on the headers and wrote a few pieces of documentation,
but this needs more work to fill out the documentation.

include/SDL3/SDL_messagebox.h

@@ -19,6 +19,12 @@
   3. This notice may not be removed or altered from any source distribution.
 */
 
+/**
+ * # CategoryMessagebox
+ *
+ * Message box support routines.
+ */
+
 #ifndef SDL_messagebox_h_
 #define SDL_messagebox_h_