[docs] Credit card chapter updates

[Jon Lapham]

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.

http://www.gnucash.org/docs/v1.8/C/gnucash-guide/chapter6.html

PS: I have not committed to CVS, I'm waiting until after the next docs 
release, unless y'all think this should go in?

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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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