doc: adding example to Buffer.isBuffer method

PR-URL: #36233
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>

doc/api/buffer.md

@@ -730,6 +730,14 @@ added: v0.1.101
 
 Returns `true` if `obj` is a `Buffer`, `false` otherwise.
 
+```js
+Buffer.isBuffer(Buffer.alloc(10)); // true
+Buffer.isBuffer(Buffer.from('foo')); // true
+Buffer.isBuffer('a string'); // false
+Buffer.isBuffer([]); // false
+Buffer.isBuffer(new Uint8Array(1024)); // false
+```
+
 ### Static method: `Buffer.isEncoding(encoding)`
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