Clarify how to define filters

[flybayer]

I was confused about how to do this, as are others.

[matthew-kerle]

Thanks for taking the time to submit a PR to update these docs. Since it had new Vue({... at the top, it was misleading and I tried to register as part of the main Vue instance as well.

[chrisvfritz]

Thanks! I made a few tweaks, particularly so that the component example is not specific to a single-file component or specific module system, but this is a great change. 🙂