document the purpose of SDL_SetTextInputRect

(cherry picked from commit 3e6513c77390969cd870726690bf89e39fe91e60)

include/SDL_keyboard.h

@@ -298,8 +298,10 @@ extern DECLSPEC void SDLCALL SDL_ClearComposition(void);
 extern DECLSPEC SDL_bool SDLCALL SDL_IsTextInputShown(void);
 
 /**
- * Set the rectangle used to type Unicode text inputs.
- *
+ * Set the rectangle used to type Unicode text inputs. Native input methods
+ * will place a window with word suggestions near it, without covering the
+ * text being inputted.
+ * 
  * To start text input in a given location, this function is intended to be
  * called before SDL_StartTextInput, although some platforms support moving
  * the rectangle even while text input (and a composition) is active.