Web.Config changes in 2005

I'm getting more wacky errors and warnings about my web.config
file in a project I upgraded from 2003.

Is there a good online resource for which elements are valid in 2005, and
what has changed since 2003?

Google is your friend, but MSDN is your lover.

http://msdn2.microsoft.com/en-us/library/ms960520.aspx

http://msdn2.microsoft.com/en-US/library/ms181993.aspx

How do I get to the Settings and Constraints editor?

Here's what that page says:

In the Settings and Constraints Editor, Web.config settings are located under Application Settings, WebApplication, and then Configuration.

I cannot find any of the keywords in that sentence.

On another page:

To open the Settings and Constraints Editor
On the View menu, point to Other Windows and choose Settings and Constraints.

In the 'Other Windows' list, there is no 'Settings and Constraints' editor.