Correct syntax for PermissionSet clause

Hello,
1. I didn't find any full documentation on web.config, and looking for some good help.
2. Specifically the PermissionSet clause - I am trying using it, but there may be problems. I need full example, where exactly should I put it
I have found this link (http://msdn.microsoft.com/en-us/library/wyts434y.aspx) but need full code.
 
Thanks
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