title: Python Bokeh tooltip text formatting
body: <p>The tooltip example presented in the reference guide show the following examples of formatting:</p>
  
  <pre><code>hover.tooltips = [
      ("index", "$index"),
      ("(x,y)", "($x, $y)"),
      ("radius", "@radius"),
      ("fill color", "$color[hex, swatch]:fill_color"),
      ("foo", "@foo"),
      ("bar", "@bar"),
      ("baz", "@baz{safe}"),
      ("total", "@total{$0,0.00}"
  </code></pre>
  
  <p>The 3 examples <code>{safe}</code>, <code>{$0,0.00}</code>  and <code>"$color[hex, swatch]:fill_color"</code> are not clear: where can i find some documentation on them?</p>
  
  <p>Basically I would like to understand what is possible and what isn't.</p>
  
  <p>At the moment (for instance) I have 1 input that us a very long string (its a newspaper article) that <strong>I would like to format so it only shows the x first characters.</strong></p>
  
  <p>Other example I have a field <code>@datetime</code> that is retrieving its value from a datetime index. At the moment the tooltip displays that value as a <code>int64</code> character. How to <strong>use a format tool such as <code>Timestamp.strftime("%d-%m-%Y")</code> so that it shows the date time in human readable format?</strong></p>
  
  <p>But I would like to have a clearer overview of what is possible/how that aspect of bokeh works</p>