Changed Shared Fonts section to not overstate browser cache disadvantages

@mkruisselbrink / @jyasskin PTAL
Maybe add
> The [`stale-while-revalidate` Cache-Control extension](http://tools.ietf.org/html/rfc5861#section-3) can allow the web page to use the font without a delay, but it still causes the new font to be downloaded while the user is actively using the network for other purposes.