docs: Filled in some category pages.

include/SDL3/SDL_error.h

@@ -23,6 +23,26 @@
  * # CategoryError
  *
  * Simple error message routines for SDL.
+ *
+ * Most apps will interface with these APIs in exactly one function: when
+ * almost any SDL function call reports failure, you can get a human-readable
+ * string of the problem from SDL_GetError().
+ *
+ * These strings are maintained per-thread, and apps are welcome to set their
+ * own errors, which is popular when building libraries on top of SDL for
+ * other apps to consume. These strings are set by calling SDL_SetError.
+ *
+ * A common usage pattern is to have a function that returns true for success
+ * and false for failure, and do this when something fails:
+ *
+ * ```c
+ * if (something_went_wrong) {
+ *    return SDL_SetError("The thing broke in this specific way: %d", errcode);
+ * }
+ * ```
+ *
+ * It's also common to just return `false` in this case if the failing thing
+ * is known to call SDL_SetError(), so errors simply propagate through.
  */
 
 #ifndef SDL_error_h_