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_clipboard.h

@@ -20,9 +20,12 @@
 */
 
 /**
- * \file SDL_clipboard.h
+ * # CategoryClipboard
  *
- * Include file for SDL clipboard handling
+ * SDL provides access to the system clipboard, both for reading information
+ * from other processes and publishing information of its own.
+ *
+ * This is not just text! SDL apps can access and publish data by mimetype.
  */
 
 #ifndef SDL_clipboard_h_