Minor improvements to TextInput docs (#9859)

packages/flutter/lib/src/services/text_input.dart

@@ -13,7 +13,7 @@ import 'text_editing.dart';
 
 export 'dart:ui' show TextAffinity;
 
-/// For which type of information to optimize the text input control.
+/// The type of information for which to optimize the text input control.
 enum TextInputType {
   /// Optimize for textual information.
   text,
@@ -28,7 +28,7 @@ enum TextInputType {
   datetime,
 }
 
-/// A action the user has requested the text input control to perform.
+/// An action the user has requested the text input control to perform.
 enum TextInputAction {
   /// Complete the text input operation.
   done,
@@ -49,7 +49,7 @@ class TextInputConfiguration {
     this.actionLabel,
   });
 
-  /// For which type of information to optimize the text input control.
+  /// The type of information for which to optimize the text input control.
   final TextInputType inputType;
 
   /// What text to display in the text input control's action button.