Improve transition rule documentation

[KronosTheLate]

This PR adds a much more complicated example for defining transition rules, as well as significantly changing the wording.

The old documentation was not enough for me to understand how to define the rules, so this took some trial and error. I believe that the documentation in the PR would have made things more clear from the start.

[FedeClaudi]

That's a great example, thank you.
I really appreciate this string of PRs, thanks for helping making Term better!

[KronosTheLate]

Hey, thank you. Term is awesome <3

Edit:
It is actually really encouraging to have these PRs reviewed and merged so quickly, and to get such positive feedback on them. So good on you for making this a project that is nice to contribute to ^_^