Update composing rect documentation (#77951)
Update composing rect documentation (#77951)

The TextInput.setMarkedRect message was originally added to support
positioning the IME candidates menu on iOS, but is now used by iOS,
Windows, macOS, and Linux desktop embedders.
flutter/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter