Add missing docs (#9696)

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

@@ -8,7 +8,7 @@ import 'package:flutter/services.dart';
 /// to provide as-you-type validation and formatting of the text being edited.
 ///
 /// Text modification should only be applied when text is being committed by the
-/// IME and not on text under composition (i.e. when 
+/// IME and not on text under composition (i.e., only when
 /// [TextEditingValue.composing] is collapsed).
 ///
 /// Concrete implementations [BlacklistingTextInputFormatter], which removes
@@ -75,7 +75,8 @@ class _SimpleTextInputFormatter extends TextInputFormatter {
 /// characters patterns.
 ///
 /// Instances of blacklisted characters found in the new [TextEditingValue]s
-/// will be replaced with the [replacementString] which defaults to ``.
+/// will be replaced with the [replacementString] which defaults to the empty
+/// string.
 ///
 /// Since this formatter only removes characters from the text, it attempts to
 /// preserve the existing [TextEditingValue.selection] to values it would now
@@ -83,15 +84,16 @@ class _SimpleTextInputFormatter extends TextInputFormatter {
 ///
 /// See also:
 ///
-///  * [TextInputFormatter].
-///  * [WhitelistingTextInputFormatter].
+///  * [WhitelistingTextInputFormatter], which uses a whitelist instead of a
+///    blacklist.
 class BlacklistingTextInputFormatter extends TextInputFormatter {
+  /// Creates a formatter that prevents the insertion of blacklisted characters patterns.
+  ///
+  /// The [blacklistedPattern] must not be null.
   BlacklistingTextInputFormatter(
-    this.blacklistedPattern, 
-    {
+    this.blacklistedPattern, {
     this.replacementString: '',
-    }
-  ) : assert(blacklistedPattern != null);
+  }) : assert(blacklistedPattern != null);
 
   /// A [Pattern] to match and replace incoming [TextEditingValue]s.
   final Pattern blacklistedPattern;
@@ -126,9 +128,12 @@ class BlacklistingTextInputFormatter extends TextInputFormatter {
 ///
 /// See also:
 ///
-///  * [TextInputFormatter].
-///  * [BlacklistingTextInputFormatter].
+///  * [BlacklistingTextInputFormatter], which uses a blacklist instead of a
+///    whitelist.
 class WhitelistingTextInputFormatter extends TextInputFormatter {
+  /// Creates a formatter that allows only the insertion of whitelisted characters patterns.
+  ///
+  /// The [blacklistedPattern] must not be null.
   WhitelistingTextInputFormatter(this.whitelistedPattern) :
     assert(whitelistedPattern != null);