Reference docs: what happens when you change the Fleet web address (#30999)

- @noahtalerman: I think changing the Fleet web address means you'll
have to re-enroll all your hosts.
- We have a [feature
request](https://github.com/fleetdm/fleet/issues/29878) to add this copy
to the UI but I think we want to get this in the docs ASAP

---------

Co-authored-by: Victor Lyuboslavsky <2685025+getvictor@users.noreply.github.com>
Co-authored-by: Rachael Shaw <r@rachael.wtf>

docs/REST API/rest-api.md

@@ -1354,6 +1354,8 @@ Modifies the Fleet's configuration with the supplied information.
 | ai_features_disabled              | boolean | Whether AI features are disabled.                                                           |
 | query_report_cap                  | integer | The maximum number of results to store per query report before the report is clipped. If increasing this cap, we recommend enabling reports for one query at time and monitoring your infrastructure. (Default: `1000`) |
 
+> Note: If `server_url` changes, hosts that enrolled to the old URL will need to re-enroll, or they will no longer communicate with Fleet. Before re-enrolling Android hosts, you'll need to turn Android MDM off and back on to point Google to the new `server_url`.
+
 <br/>
 
 ##### Example request body