docs: added new use case for Register.after_filter

[tmbb]

This commit updates the docs of Register.after_filter. It explains how to use this filter to add new users to a web app using ExAdmin.

While this might a be a little too specific, it is a common use case. The documentation on the filter (while providing a helpful code example) could use some prose describing why you would want to use it in practice.

[tmbb]

The build fails but it seems to be some kind of problem with the elixir environment. My changes are only in the @doc attribute.