docs: add documentation

Author: funwithflutter

lib/src/presentation/widgets/sign_in_form.dart

@@ -3,6 +3,11 @@ import 'package:provider/provider.dart';
 
 import '../../application/auth/sign_in_handler/sign_in_handler_state.dart';
 
+/// A wrapper for a standard [Form] widget. It allows the [Form] to respond to
+/// error messages if the current email and password fields are not correct.
+///
+/// Should be used in conjunction with [EmailTextFormField] and
+/// [PasswordTextFormField]
 class SignInForm extends StatelessWidget {
   const SignInForm({Key key, @required this.child})
       : assert(child != null),