docs: validation for custom methods

[AshotN]

Summary

This PR expands the documentation on custom methods. Mainly an example on how to use validators with custom methods.

[daffl]

Thank you! I think we should move this to the validators documentation at https://feathersjs.com/api/schema/validators.html but then we can merge.

[marshallswain]

Thanks @AshotN for enabling PR branch editing. I've pushed up the change that @daffl requested, so this should be ready to merge.