[Fleet] Document serverless restrictions changes

[jillguyonnet]

Issue: elastic/kibana#165672
PR: elastic/kibana#175315

Details

The above change lifted some existing restrictions in serverless Fleet. It is now possible to:

• create proxy configurations
• create new Fleet Server configurations with a proxy
• create new Elasticsearch outputs with a proxy

The following restrictions exist:

• The host URL for new Fleet Server configurations cannot be configured through the UI or API.
• The host URL for new Elasticsearch outputs cannot be configured through the UI or API.

Suggested documentation change

Edit the Fleet and Elastic Agent restrictions for Elastic Cloud serverless page with the following:

• Fleet Server section: add a paragraph stating that it is possible to create a Fleet Server configuration with a proxy and that the host URL is fixed.
• New Outputs section mentioning that it is possible to create a new Elasticsearch output with a proxy and that the host URL is fixed.