Update CircularDependencyError docs to reference use_alter

[Former user]

I just ran into a CircularDependencyError which turned out to be simply due to two tables referencing each other.

After some digging I found this thread, which instantly gave me a solution:

http://www.mail-archive.com/[e-mail]/msg24991.html

This investigation could have been shorter if the CircularDependencyError docs had a short explanation and/or a quick link to the (great) use_alter explanation here:

http://www.sqlalchemy.org/docs/core/schema.html?highlight=use_alter#creating-dropping-foreign-key-constraints-via-alter

[Mike Bayer]

good idea, this is in 9d842790b40717d35195d7ed23dd1c151430c347 . will be up on the site in the next doc build.

[Mike Bayer]

Removing milestone: 0.7.3 (automated comment)