doc,url: clarify domainTo* when built without ICU

Continuation of: #35099

Signed-off-by: Darshan Sen <raisinten@gmail.com>

PR-URL: #38789
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

doc/api/url.md

@@ -942,6 +942,9 @@ invalid domain, the empty string is returned.
 
 It performs the inverse operation to [`url.domainToUnicode()`][].
 
+This feature is only available if the `node` executable was compiled with
+[ICU][] enabled. If not, the domain names are passed through unchanged.
+
 ```mjs
 import url from 'url';
 
@@ -979,6 +982,9 @@ domain, the empty string is returned.
 
 It performs the inverse operation to [`url.domainToASCII()`][].
 
+This feature is only available if the `node` executable was compiled with
+[ICU][] enabled. If not, the domain names are passed through unchanged.
+
 ```mjs
 import url from 'url';