title: Explanation and all options for $options in Symfony form builder
body: <p>I have tried a lot and could be able to find what are the various available options which i can use for <code>$options</code> in array for form builder.</p>
  
  <p>I know that options are different for</p>
  
  <ol>
  <li>fields</li>
  <li>entity</li>
  <li>collection</li>
  </ol>
  
  <p>But could not be able to find what are those for these categories and their explanations</p>
  
  <pre><code>-&gt;add('name','text',array());
  -&gt;add('user','entity',array());
  -&gt;add('user','collection',array());
  </code></pre>
  
  <p>Is there any resource which can tell what options i can use in above cases</p>