title: Why cant I setRetainInstance(true) for nested fragment?
body: <p>As I understand, It means that I must use <code>Bundle instance</code> to store fragment's data. But what is a purpose of this limitation?</p>
  
  <pre><code>public void setRetainInstance(boolean retain) {
      if (retain &amp;&amp; mParentFragment != null) {
          throw new IllegalStateException(
                  "Can't retain fragements that are nested in other fragments");
      }
      mRetainInstance = retain;
  }
  </code></pre>