[docs] Credit card chapter updates

[Jon Lapham]

doug foskey wrote:
> That is not as silly as it sounds. Debit cards are usually a bank account, 
> where you draw from your money. Credit cards are a loan. (so one asset & 
> t'other liability.)

Who said it was silly?

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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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