document how the FacetsFilter class works

Problem/Motivation

It would help with debugging and DX if the FacetsFilter class's class docs gave an overview of how it works.

For instance:

- it defines a facet config entity on the fly that is not saved (AFAICT)
- the ID of that entity is derived from the view as folllows:

> 'facet_source_id' => 'search_api:views_' . $this->displayHandler->getPluginId() . '__' . $this->view->id() . '__' . $this->view->current_display,

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes