Add missing parameters and return types to SDL2 header docs (#13162)

include/SDL_keyboard.h

@@ -298,6 +298,9 @@ extern DECLSPEC void SDLCALL SDL_ClearComposition(void);
 /**
  * Returns if an IME Composite or Candidate window is currently shown.
  *
+ * \returns SDL_TRUE if an IME Composite or Candidate window is currently
+ *          shown else SDL_FALSE.
+ *
  * \since This function is available since SDL 2.0.22.
  */
 extern DECLSPEC SDL_bool SDLCALL SDL_IsTextInputShown(void);