doc: document buffer.kStringMaxLength

PR-URL: #38688
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/buffer.md

@@ -3343,6 +3343,15 @@ added: v3.0.0
 
 An alias for [`buffer.constants.MAX_LENGTH`][].
 
+### `buffer.kStringMaxLength`
+<!-- YAML
+added: v3.0.0
+-->
+
+* {integer} The largest length allowed for a single `string` instance.
+
+An alias for [`buffer.constants.MAX_STRING_LENGTH`][].
+
 ### `buffer.transcode(source, fromEnc, toEnc)`
 <!-- YAML
 added: v7.1.0