Add missing \returns, change "return" to "returns" to have same naming

include/SDL3/SDL_keyboard.h

@@ -293,6 +293,8 @@ extern DECLSPEC void SDLCALL SDL_ClearComposition(void);
 /**
  * Returns if an IME Composite or Candidate window is currently shown.
  *
+ * \returns SDL_TRUE if shown, else SDL_FALSE
+ *
  * \since This function is available since SDL 3.0.0.
  */
 extern DECLSPEC SDL_bool SDLCALL SDL_TextInputShown(void);