[css3-fonts] updated description of FontLoader interface

Based on feedback from ms2ger [1] and others, I've updated the CSS3 Fonts
editor's draft to include a better normative description of the
behavior of the FontLoader object.  I'm still not entirely happy with
the prose, it's still a bit of a jumble of helpful description for
authors mixed with implementorese but I think it's clear enough for
now.

I've noted several issues I came across, some fairly simple and one
slightly complex having to do with a fallback case.

Regards,

John Daggett

[1] http://lists.w3.org/Archives/Public/www-style/2012Nov/0305.html