title: How does Builders<TA>.Update.Set() handle parallel access?
body: <p>Assuming 'items' (provided as a parameter in the Set() method below) is defined as List (TM is some structure type, i.e. made of properties only) containing <strong>only one item</strong>, and 10 processes trying to write simultaneously an update to <code>items</code> (<strong>without</strong> adding anything to the List):</p>
  
  <pre><code>Builders&lt;TK&gt;.Update.Set("Items", items);
  </code></pre>
  
  <p>An update means that there is at least one element in the BSON with a modified <strong>value</strong>.</p>
  
  <p>What will MongoDB in such case?</p>
  
  <p>Will it eventually overwrite with the last process into that <strong>single</strong> element (leaving that List with a single element)?</p>
  
  <p>Or will it <strong>add</strong> 10 such elements (items) into that List?</p>