[Plone-docs] updated plone 4 manual on plone.org with expiration/publication dates

[Kees H. <kee...@gm...>]

FYI: In 
http://plone.org/documentation/manual/plone-4-user-manual/adding-content/setting-basic-properties, 
i added the text from 
http://keeshink.blogspot.de/2009/07/how-do-plones-expirationpublication.html

It's just a summary for beginning Plone users on how these dates work...