rss_converter_twitter.com.xsl: explain why the style attribute is removed

Since commit 6817108 (rss_converter_twitter.com.xsl: strip the style
attribute from HTML elements, 2017-06-08) the twitter.com stylesheet
removes the "style" attribute from elements when copying them.

This is in order to create a more visually neutral output, but also
because the style attribute may even contain dangerous content:
https://validator.w3.org/feed/docs/warning/DangerousStyleAttr.html

However someone who reads the code may not be familiar with (or have
forgotten) why this is done, so explain that in a comment to avoid them
the burden of digging in the project history.