Document how to use scrollTop() properly

Via discussion in #14515.

This should be added to scrollTop():

Note that scrollTop can be used to read, but not write, the document scroll position. To scroll the document, animated or not, you have to call `scrollTop()` on the body element:

<pre><code>
// Without animation
$( document.body ).scrollTop( 500 );

// With animation
$( document.body ).animate({
  scrollTop: 500
});
</code></pre>

Any examples elsewhere that suggest $( "html, body" ).animate({ scrollTop: ... }); are wrong, since this will cause the animation callback to get triggered twice.

Update entries/scrollTop.xml with the documented distinction between reading and writing the document scroll position, including the provided examples. Then review other examples that suggest using $("html, body").animate({ scrollTop: ... }); and correct them if needed. Done means the scrollTop() documentation reflects the requested guidance and no conflicting examples remain.