Heavy work on improving category documentation.

Still more to go!

Reference Issue #9440.

include/SDL3/SDL_cpuinfo.h

@@ -29,6 +29,11 @@
  * 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.
+ *
+ * CPU instruction set checks, like SDL_HasSSE() and SDL_HasNEON(), are
+ * available on all platforms, even if they don't make sense (an ARM processor
+ * will never have SSE and an x86 processor will never have NEON, for example,
+ * but these functions still exist and will simply return false in these cases).
  */
 
 #ifndef SDL_cpuinfo_h_