doc: improve checkServerIdentity docs

PR-URL: #17203
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/tls.md

@@ -808,8 +808,9 @@ added: v0.11.3
   * `servername`: {string} Server name for the SNI (Server Name Indication) TLS
     extension.
   * `checkServerIdentity(servername, cert)` {Function} A callback function
-    to be used when checking the server's hostname against the certificate.
-    This should throw an error if verification fails. The method should return
+    to be used (instead of the builtin `tls.checkServerIdentity()` function)
+    when checking the server's hostname against the certificate.
+    This should return an {Error} if verification fails. The method should return
     `undefined` if the `servername` and `cert` are verified.
   * `session` {Buffer} A `Buffer` instance, containing TLS session.
   * `minDHSize` {number} Minimum size of the DH parameter in bits to accept a