doc: explicitly document highWaterMark option

PR-URL: https://github.com/nodejs/node/pull/17049
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/fs.md

@@ -865,6 +865,7 @@ changes:
   * `autoClose` {boolean}
   * `start` {integer}
   * `end` {integer}
+  * `highWaterMark` {integer}
 
 Returns a new [`ReadStream`][] object. (See [Readable Stream][]).
 
@@ -880,7 +881,8 @@ const defaults = {
   encoding: null,
   fd: null,
   mode: 0o666,
-  autoClose: true
+  autoClose: true,
+  highWaterMark: 64 * 1024
 };
 ```