Docs: Clarifies requirements in order for Auto Backup to work in API 23.

<p>

  Users frequently invest time and effort to configure apps just the way they like them. Switching

  to a new device can cancel out all that careful configuration. Devices running Android 6.0 and

  higher automatically back up app data to the cloud. The system performs this automatic backup

  to a new device can cancel out all that careful configuration. For apps whose <a href=

  "{@docRoot}guide/topics/manifest/uses-sdk-element.html#target">target SDK version</a>

  is Android 6.0 (API level 23) and higher, devices running Android 6.0 and higher automatically

  back up app data to the cloud. The system performs this automatic backup

  for nearly all app data by default, and does so without your having to write any additional app

  code.

</p>

<p class="note">

<strong>Note:</strong> To protect user privacy, the device user must have opted in to Google

services for Auto Backup to work. The Google services opt-in dialog appears when the user goes

through the Setup Wizard or configures the first Google account on the device.

</p>

<p>

  When a user installs your app on

  a new device, or reinstalls your app on one (for example, after a factory reset), the system

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

        xmlns:tools="http://schemas.android.com/tools"

        package="com.my.appexample">

    <uses-sdk android:minSdkVersion="MNC"/>

    <uses-sdk android:targetSdkVersion="MNC"/>

    <uses-sdk android:minSdkVersion="23"/>

    <uses-sdk android:targetSdkVersion="23"/>

    <application ...

<strong>        android:fullBackupContent="&#64;xml/mybackupscheme"&gt;</strong>

    </app>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

        xmlns:tools="http://schemas.android.com/tools"

        package="com.my.appexample">

    <uses-sdk android:minSdkVersion="MNC"/>

    <uses-sdk android:targetSdkVersion="MNC"/>

    <uses-sdk android:minSdkVersion="23"/>

    <uses-sdk android:targetSdkVersion="23"/>

    <application ...

<strong>        android:allowBackup="false"&gt;</strong>

    </application>