title: Confusion with Angular component property binding
body: <p>I'm using the PrimeNG UI components on an Angular 5 application and I have a little confusion about they way to bind values to some of their properties. I'll explain it with an example:</p>
  
  <p>According to the documentation (<a href="https://www.primefaces.org/primeng/#/accordion" rel="nofollow noreferrer">https://www.primefaces.org/primeng/#/accordion</a>), these are the properties for AccordionTab:</p>
  
  <pre><code>Name       Type     Default   Description
  header     string   null      Title of the tab.
  selected   boolean  false     Defines if the tab is active.
  disabled   boolean  false     Defines whether the tab can be selected
  </code></pre>
  
  <p>And then, in the code, we have the following example:</p>
  
  <pre><code>&lt;p-accordion&gt;
    &lt;p-accordionTab header="Header 1" [selected]="true"&gt;
      Content 1
    &lt;/p-accordionTab&gt;
    &lt;p-accordionTab header="Header 2"&gt;
      Content 2
    &lt;/p-accordionTab&gt;
    &lt;p-accordionTab header="Header 3"&gt;
      Content 3    
    &lt;/p-accordionTab&gt;
  &lt;/p-accordion&gt;
  </code></pre>
  
  <p>As you can see, <strong>header</strong> is used without brackets, while <strong>selected</strong> does use them. Please correct me if I'm wrong, but brackets are used to bind values to the component properties, while a literal without brackets refers to a directive, right?</p>
  
  <p>Why are they used differently in this case?</p>
  
  <p>Thanks in advance!</p>