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_intrin.h

@@ -19,9 +19,7 @@
   3. This notice may not be removed or altered from any source distribution.
 */
 
-/**
- *  \file SDL_intrin.h
- *
+/*
  *  Header file for CPU intrinsics for SDL
  */