doc: describe SNI host name format

Ref: #23329

doc/api/tls.md

@@ -894,7 +894,7 @@ changes:
     protocol name. Passing an array is usually much simpler, e.g.
     `['hello', 'world']`.
   * `servername`: {string} Server name for the SNI (Server Name Indication) TLS
-    extension.
+    extension. It must be a host name, and not an IP address.
   * `checkServerIdentity(servername, cert)` {Function} A callback function
     to be used (instead of the builtin `tls.checkServerIdentity()` function)
     when checking the server's hostname (or the provided `servername` when