[docs] Credit card chapter updates

[David Kågedal]

Jon Lapham <lapham at extracta.com.br> writes:

> I updated the credit card chapter of the documentation, we now have
> real screenshots!  Please, if you have a few minutes, read through it
> and send me recommendations.

I wouldn't write the first sentence like this:

    Since you probably write a check or make an electronic payment to
    the credit card company each month [...]

But that's probably because most people here in don't use credit
cards.  Using cards that withdraw money directly from your account is
much more common.

Don't assume to much about your user, just tell her how it works.

-- 
David Kågedal