title: User permissions in Keystone.js Admin UI
body: <p>I am considering migrating an <code>express.js</code> app to <code>keystone.js</code> . The app serves an API (no pages) so we're interested in the admin ui and rich field capacities of Keystone.</p>
  
  <p>We have a specific requirement I am trying to implement: we have users with documents. It is a many to many relationship. 
  A user can have several documents and a document can be managed by several users. 
  A user can only edit his/her documents.  I am able to show a user's documents in the UI but not to restrict the access to those documents. The canAccessKeystone field grants admin permissions on all content.</p>
  
  <p>Can I achieve that and if yes, how?</p>