Mention codemod -x flag in docs

[kiri11]

Summary

After #879 got implemented, we can assume that most user codemods are loaded in this way instead of editing the LibCST source (it would be just pip installed). So -x flag is very important and should be mentioned in docs.

I am open to suggestions on how to improve the wording and also mention the built-in codemods and libcst.tool list better.

Test Plan

#shipit (just docs)

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[zsol]

Sounds good, thanks! Note that you could always have loaded Codemods from arbitrary locations as long as you had a config file in the root of your repo and then listed the locations of the Codemods there.

But yeah with -x even that is not necessary anymore.