[shared_preferences] Add missing DartDoc (#2296)

Also adds a lint to prevent further undocumented methods.
diff --git a/packages/shared_preferences/CHANGELOG.md b/packages/shared_preferences/CHANGELOG.md
index 1cc5c51..083d2b9 100644
--- a/packages/shared_preferences/CHANGELOG.md
+++ b/packages/shared_preferences/CHANGELOG.md
@@ -1,7 +1,11 @@
+## 0.5.4+6
+
+* Add missing documentation and a lint to prevent further undocumented APIs.
+
 ## 0.5.4+5
 
-* Update and migrate iOS example project by removing flutter_assets, change 
-  "English" to "en", remove extraneous xcconfigs and framework outputs, 
+* Update and migrate iOS example project by removing flutter_assets, change
+  "English" to "en", remove extraneous xcconfigs and framework outputs,
   update to Xcode 11 build settings, and remove ARCHS.
 
 ## 0.5.4+4
diff --git a/packages/shared_preferences/analysis_options.yaml b/packages/shared_preferences/analysis_options.yaml
new file mode 100644
index 0000000..4d3c53a
--- /dev/null
+++ b/packages/shared_preferences/analysis_options.yaml
@@ -0,0 +1,11 @@
+# This exists to add a lint for missing API docs just on this specific package,
+# since not all packages have coverage for all their public members yet and
+# adding it in would be non-trivial. `public_member_api_docs` should be applied
+# to new packages going forward, and ideally the main `analysis_options.yaml`
+# file as soon as possible.
+
+include: ../../analysis_options.yaml
+
+linter:
+  rules:
+    - public_member_api_docs
diff --git a/packages/shared_preferences/example/lib/main.dart b/packages/shared_preferences/example/lib/main.dart
index 0d93675..5c7f0b0 100644
--- a/packages/shared_preferences/example/lib/main.dart
+++ b/packages/shared_preferences/example/lib/main.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// ignore_for_file: public_member_api_docs
+
 import 'dart:async';
 
 import 'package:flutter/material.dart';
diff --git a/packages/shared_preferences/lib/shared_preferences.dart b/packages/shared_preferences/lib/shared_preferences.dart
index 3fe176f..c82116a 100644
--- a/packages/shared_preferences/lib/shared_preferences.dart
+++ b/packages/shared_preferences/lib/shared_preferences.dart
@@ -19,6 +19,11 @@
 
   static const String _prefix = 'flutter.';
   static Completer<SharedPreferences> _completer;
+
+  /// Loads and parses the [SharedPreferences] for this app from disk.
+  ///
+  /// Because this is reading from disk, it shouldn't be awaited in
+  /// performance-sensitive blocks.
   static Future<SharedPreferences> getInstance() async {
     if (_completer == null) {
       _completer = Completer<SharedPreferences>();
diff --git a/packages/shared_preferences/pubspec.yaml b/packages/shared_preferences/pubspec.yaml
index dfce291..89326d9 100644
--- a/packages/shared_preferences/pubspec.yaml
+++ b/packages/shared_preferences/pubspec.yaml
@@ -3,7 +3,7 @@
   Wraps NSUserDefaults on iOS and SharedPreferences on Android.
 author: Flutter Team <flutter-dev@googlegroups.com>
 homepage: https://github.com/flutter/plugins/tree/master/packages/shared_preferences
-version: 0.5.4+5
+version: 0.5.4+6
 
 flutter:
   plugin: