Explain the groups pattern field + documentation

Problem/Motivation

It's not very clear from the interface that the pattern field is building a REGEX in the background for you. An explanation and a few use cases in the form would help a lot.

Let's also take the opportunity here to create/update a documentation page for this.

Proposed resolution

Add a short description + link to the documentation page. I think the description can go below the fieldset as there is no need to explain this for every pattern field that might be added to the rules.

Remaining tasks

Create pull request

User interface changes

Pattern field is explained in the UI.

API changes

N/A

Data model changes

N/A