docs: fix broken link, reduce API info about backup and point to dev guide

<h4 id="backup-manager">Backup manager</h4>

<p>The platform now provides a generalized backup manager facility that

<p>The platform now provides a generalized backup service that

applications can use to backup and restore user data, to ensure that users can

maintain their data when switching devices or reinstalling the application. The

backup manager handles the work of transporting the application data to and from

the backup storage area in the cloud. The backup manager can store any type of

Backup Manager handles the work of transporting the application data to and from

the backup storage area in the cloud. The Backup Manager can store any type of

data, from arbitrary data to files, and manages backup and restore operations

in an atomic manner. </p>

<p>Any application can use the backup manager to save and restore data. To do

so, the application instantiates a {@link android.app.backup.BackupManager} and

uses its methods to trigger a new backup or restore operation. The application

must also create a subclass of {@link android.app.backup.BackupAgent} and

implement its methods to handle calls from the backup manager to get data for

backup and provide data to restore

({@link android.app.backup.BackupAgent#onBackup(android.os.ParcelFileDescriptor, android.app.backup.BackupDataOutput, android.os.ParcelFileDescriptor) onBackup()}

and {@link android.app.backup.BackupAgent#onRestore(android.app.backup.BackupDataInput, int, android.os.ParcelFileDescriptor) onRestore()}).

A simple implementation of a BackupAgent useful for backing up preferences 

and files is available by using {@link android.app.backup.BackupAgentHelper}.

For more information, see <a

href="{@docRoot}guide/topics/topics/data/backup.html">Data Backup</a>.</p>

in an atomic manner. For more information, see <a

href="{@docRoot}guide/topics/data/backup.html">Data Backup</a>.</p>

<h4>Graphics</h4>