Documented how to use system service dumper.

Test: m services
Bug: 163921395

Change-Id: I87cde223291b1a6a86ba66d391832d8eef95bfa9

services/core/java/com/android/server/Dumpable.java

@@ -21,6 +21,8 @@ import android.util.IndentingPrintWriter;
 
 /**
  * Interface used to dump {@link SystemServer} state that is not associated with any service.
+ *
+ * <p>See {@link SystemServer.SystemServerDumper} for usage example.
  */
 public interface Dumpable {
 

services/java/com/android/server/SystemServer.java

@@ -645,6 +645,21 @@ public final class SystemServer implements Dumpable {
         TimeUtils.formatDuration(mRuntimeStartElapsedTime, pw); pw.println();
     }
 
+    /**
+     * Service used to dump {@link SystemServer} state that is not associated with any service.
+     *
+     * <p>To dump all services:
+     *
+     * <pre><code>adb shell dumpsys system_server_dumper</code></pre>
+     *
+     * <p>To get a list of all services:
+     *
+     * <pre><code>adb shell dumpsys system_server_dumper --list</code></pre>
+     *
+     * <p>To dump a specific service (use {@code --list} above to get service names):
+     *
+     * <pre><code>adb shell dumpsys system_server_dumper --name NAME</code></pre>
+     */
     private final class SystemServerDumper extends Binder {
 
         @GuardedBy("mDumpables")