Also show code callouts as tooltips

[florent-leborgne]

In case of long code blocks with callouts, it'd be super handy to have tooltips on callout numbers that include the text of the callout.

Here is an example of a long code block. Instead of scrolling, the user could just hover over each callout and read what it means.

[theletterf]

+CC @itsalexcm