title: How textScore field is calculated in mongodb full text search?
body: <p>I want to know how MongoDB is calculating the score of the text in full text search.
  Like if I will search for <code>samsung note edge</code> in followings:</p>
  
  <pre><code>Samsung Galaxy Note Edge
  Samsung Galaxy Note 4
  Samsung Galaxy S6 Edge
  Samsung Galaxy Note 4 duos
  Samsung Z
  </code></pre>
  
  <p>Full text Search as follows:</p>
  
  <pre><code>db.mobiles.find({
      $text : {$search : "samsung note edge"}
  }, {
      score : {$meta : "textScore" }
  }).sort({
      score : {$meta : "textScore" }
  })
  </code></pre>
  
  <p>Is giving me result as follows:</p>
  
  <pre><code>{
      name : "Samsung Galaxy Note Edge",
      score: 1.875000
  },
  {
      name : "Samsung Galaxy Note 4",
      score: 1.250000
  },
  {
      name : "Samsung Galaxy S6 Edge",
      score: 1.250000
  },
  {
      name : "Samsung Galaxy Note 4 duos",
      score: 1.200000
  },
  {
      name : "Samsung Z",
      score: 0.750000
  }
  </code></pre>
  
  <p>The results are different if I will search for <code>Samsung edge</code></p>