[Request] Improve Fleet preconfig docs

## Description

*(Describe what needs to be documented. Link to relevant issues.)*

The [existing docs](https://www.elastic.co/guide/en/kibana/master/fleet-settings-kb.html) for Fleet preconfig don't do a good job of explaining the intended behavior. We should better describe what changes are made in the case that `is_managed` is true or false, when the user adds, edits, or deletes resources.

We also need to decide how to address exceptions, such as fields that are defined in preconfig but are not frozen and still user-editable.

## Collaboration
*(Choose the expected collaboration model and delete the others.)*

- The product team will provide the initial content and the docs team will edit / review

*(Assign a contact person for this issue. We need to have a contact person in the product/development team to provide information about how the item to be documented works. This can be omitted when the product/development team is providing the initial content, as the contact person will be the one making the initial contribution.)*

**Contact Person:**
@joshdover

## Suggested Target Release

*(Let us know if this feature is needed for a specific release.)*

## Stakeholders

*(Please list any stakeholders for this issue.)*

@mostlyjason