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_cpuinfo.h

@@ -22,9 +22,13 @@
 /* WIKI CATEGORY: CPUInfo */
 
 /**
- *  \file SDL_cpuinfo.h
+ * # CategoryCPUInfo
  *
- *  CPU feature detection for SDL.
+ * CPU feature detection for SDL.
+ *
+ * These functions are largely concerned with reporting if the system has
+ * access to various SIMD instruction sets, but also has other important info
+ * to share, such as system RAM size and number of logical CPU cores.
  */
 
 #ifndef SDL_cpuinfo_h_