title: What does withReplacement do, if specified for sample against a Spark Dataframe
body: <p>Reading the spark documentation: <a href="http://spark.apache.org/docs/2.1.0/api/python/pyspark.sql.html#pyspark.sql.DataFrame.sample" rel="noreferrer">http://spark.apache.org/docs/2.1.0/api/python/pyspark.sql.html#pyspark.sql.DataFrame.sample</a></p>
  
  <p>There is this boolean parameter <code>withReplacement</code> without much explanation. </p>
  
  <blockquote>
    <p>sample(withReplacement, fraction, seed=None)</p>
  </blockquote>
  
  <p>What is it and how do we use it? </p>