docs(annotations): note alias annotation is limited to A, AAAA, and CNAME

Signed-off-by: u-kai <76635578+u-kai@users.noreply.github.com>

Author: u-kai

docs/annotations/annotations.md

@@ -305,6 +305,9 @@ Additional annotations that are currently implemented only by AWS are:
 If the value of this annotation is `true`, specifies that CNAME records generated by the
 resource should instead be alias records.
 
+This annotation is only supported on A, AAAA, and CNAME record types. Endpoints with other
+record types (e.g. MX, SRV, TXT) that have this annotation set will be rejected.
+
 This annotation is only relevant if the `--aws-prefer-cname` flag is specified.
 
 ### external-dns.alpha.kubernetes.io/set-identifier