SoundPool.Builder: document default values

Complete javadoc for android.media.SoundPool.Builder to document
 what the default values are.

Bug 17059255

Change-Id: I966e9ed00ed75a78c9b4741b7f68bae996442cdf
diff --git a/media/java/android/media/SoundPool.java b/media/java/android/media/SoundPool.java
index 9b9c767..4f74bdd 100644
--- a/media/java/android/media/SoundPool.java
+++ b/media/java/android/media/SoundPool.java
@@ -151,6 +151,10 @@
 
         /**
          * Constructs a new Builder with the defaults format values.
+         * If not provided, the maximum number of streams is 1 (see {@link #setMaxStreams(int)} to
+         * change it), and the audio attributes have a usage value of
+         * {@link AudioAttributes#USAGE_MEDIA} (see {@link #setAudioAttributes(AudioAttributes)} to
+         * change them).
          */
         public Builder() {
         }