docs: Filled in some category pages.

include/SDL3/SDL_init.h

@@ -22,10 +22,30 @@
 /**
  * # CategoryInit
  *
- * SDL subsystem init and quit functions.
+ * All SDL programs need to initialize the library before starting to work
+ * with it.
+ *
+ * Almost everything can simply call SDL_Init() near startup, with a handful
+ * of flags to specify subsystems to touch. These are here to make sure SDL
+ * does not even attempt to touch low-level pieces of the operating system
+ * that you don't intend to use. For example, you might be using SDL for
+ * video and input but chose an external library for audio, and in this case
+ * you would just need to leave off the `SDL_INIT_AUDIO` flag to make sure
+ * that external library has complete control.
+ *
+ * Most apps, when terminating, should call SDL_Quit(). This will clean up
+ * (nearly) everything that SDL might have allocated, and crucially, it'll
+ * make sure that the display's resolution is back to what the user expects
+ * if you had previously changed it for your game.
+ *
+ * SDL3 apps are strongly encouraged to call SDL_SetAppMetadata at startup
+ * to fill in details about the program. This is completely optional, but it
+ * helps in small ways (we can provide an About dialog box for the macOS menu,
+ * we can name the app in the system's audio mixer, etc). Those that want to
+ * provide a _lot_ of information should look at the more-detailed
+ * SDL_SetAppMetadataProperty().
  */
 
-
 #ifndef SDL_init_h_
 #define SDL_init_h_