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_close_code.h

@@ -19,11 +19,9 @@
   3. This notice may not be removed or altered from any source distribution.
 */
 
-/**
- *  \file SDL_close_code.h
- *
- *  This file reverses the effects of SDL_begin_code.h and should be included
- *  after you finish any function and structure declarations in your headers
+/*
+ * This file reverses the effects of SDL_begin_code.h and should be included
+ * after you finish any function and structure declarations in your headers
  */
 
 #ifndef SDL_begin_code_h