title: What is the purpose of boolean values for the breakpoint props (xs, sm, md...) in material-ui
body: <p>I am struggling to find documentation on (or results via experimenting with code) what the purpose is of having booleans as values for breakpoint props for the grid component in material-ui.
  Consulting the <a href="https://material-ui.com/api/grid/" rel="noreferrer">grid api documentation</a> reveals that booleans are valid values for the breakpoint props lg, md, sm, xl, xs.</p>
  
  <p>I understand that if I say <code>sm={3}</code> I will get a component that changes to take up 3 grid column units once the display width increases beyond the xs breakpoint (600px) but have no idea about passing a boolean as a value:</p>
  
  <p>For example what would be the reason for providing <code>xs={true}</code> or <code>md={false}</code> be?
  And how might I have learned the reason on my own? (is there some fundamental knowledge I'm lacking?)</p>