wmllint: change "usage of" magic comment to re.search

There seems no particular reason to require that this magic comment be at
the very beginning of the line, so why not switch from re.match to
re.search.

Also, update comments to reflect the fact that UtBS no longer uses this
magic comment.

A suggestion has been made to get rid of this magic comment now that UtBS
is no longer using it, but it may still be in use in some UMC somewhere.
html.py: format add-on description for HTML

The description text does not get rendered very well on a webpage. One
solution might be to use pre-wrap/word-wrap in the CSS, but due to
differences between browsers, that's a can of worms (at least for me, I'm
not a web pro).

So, the not-so-elegant solution is to add <br/> to every line.

URLs are also not linked in the plain text. Although in modern browsers
you can select the text and right-click, it's still convenient to turn
them into actual links.