Available security information section clarification

I've edited the "Available security information" section in accordance
with the discussion that generated ACTION-157. In particular, I've added
some preamble text describing the structure of the section and broadened
the "Provided by HTML" section into "Provided by web content". I've also
added an entry to "Provided by user agent" for "Has the page completed
rendering?" This last item comes out of the white text on a white
background case that results from failing to fetch a stylesheet.

See:

http://www.w3.org/2006/WSC/drafts/note/#available

Tyler