docs: clarify hostname annotation behavior (kubernetes-sigs#5912)

* docs: clarify hostname annotation behavior

* docs: specify how hostname annotation behavior can be configured

Co-authored-by: Michel Loiseleur <[email protected]>

---------

Co-authored-by: Michel Loiseleur <[email protected]>

Author: PseudoResonance

docs/annotations/annotations.md

@@ -64,7 +64,7 @@ Otherwise, use the `IP` of each of the `Service`'s `Endpoints`'s `Addresses`.
 
 ## external-dns.alpha.kubernetes.io/hostname
 
-Specifies the domain for the resource's DNS records.
+Specifies additional domains for the resource's DNS records.
 
 Multiple hostnames can be specified through a comma-separated list, e.g.
 `svc.mydomain1.com,svc.mydomain2.com`.
@@ -73,7 +73,8 @@ For `Pods`, uses the `Pod`'s `Status.PodIP`, unless they are `hostNetwork: true`
 
 Notes:
 
-- This annotation `overrides` any automatically derived hostnames (e.g., from Ingress.spec.rules[].host).
+- This annotation can override or add extra hostnames alongside any automatically derived hostnames (e.g., from Ingress.spec.rules[].host).
+- The [`ingress-hostname-source`](#external-dnsalphakubernetesioingress-hostname-source) annotation may be used to specify where to get the domain for an `Ingress` resource.
 - Hostnames must match the domain filter set in ExternalDNS (e.g., --domain-filter=example.com).
 - This is an alpha annotation — subject to change; newer versions may support alternatives or deprecate it.
 - This annotation is helpful for: