[docs] Credit card chapter updates

[Jon Lapham]

Chris Lyttle wrote:
> Jon,
> 
> Please commit any updates to docs you have done before sunday evening
> PST. This is a new release, even tho the reason for it is a bugfix, I
> still would want to commit any docs changes you had made in the
> meantime.

Committed to CVS, with some modifications sent to me by "tripp+gnucash".

Oh, and I ran xmllint on the files... it looks good.  :)

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  Jon Lapham  <lapham at extracta.com.br>          Rio de Janeiro, Brasil
  Work: Extracta Moléculas Naturais SA     http://www.extracta.com.br/
  Web: http://www.jandr.org/
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