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_blendmode.h

@@ -20,9 +20,11 @@
 */
 
 /**
- *  \file SDL_blendmode.h
+ * # CategoryBlendmode
  *
- *  Header file declaring the SDL_BlendMode enumeration
+ * Blend modes decide how two colors will mix together. There are both
+ * standard modes for basic needs and a means to create custom modes,
+ * dictating what sort of math to do what on what color components.
  */
 
 #ifndef SDL_blendmode_h_