doc: add buf.indexOf encoding arg with example

doc/api/buffer.markdown

@@ -490,17 +490,21 @@ Buffer. The method returns a reference to the Buffer so calls can be chained.
     console.log(b.toString());
       // Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
 
-### buf.indexOf(value[, byteOffset])
+### buf.indexOf(value[, byteOffset][, encoding])
 
 * `value` String, Buffer or Number
 * `byteOffset` Number, Optional, Default: 0
+* `encoding` String, Optional, Default: `utf8`
 * Return: Number
 
 Operates similar to [`Array#indexOf()`][] in that it returns either the
 starting index position of `value` in Buffer or `-1` if the Buffer does not
-contain `value`. The `value` can be a String, Buffer or Number. Strings are
-interpreted as UTF8. Buffers will use the entire Buffer (to compare a partial
-Buffer use [`Buffer#slice()`][]). Numbers can range from 0 to 255.
+contain `value`.
+The `value` can be a String, Buffer or Number.
+Strings are by default interpreted as UTF8.
+Buffers will use the entire Buffer (to compare a partial
+Buffer use [`Buffer#slice()`][]).
+Numbers can range from 0 to 255.
 
     const buf = new Buffer('this is a buffer');
 
@@ -517,6 +521,11 @@ Buffer use [`Buffer#slice()`][]). Numbers can range from 0 to 255.
     buf.indexOf(new Buffer('a buffer example').slice(0,8));
       // returns 8
 
+    const utf16Buffer = new Buffer('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');
+
+    assert.equal(4, utf16Buffer.indexOf('\u03a3',  0, 'ucs2'), 'First Sigma');
+    assert.equal(6, utf16Buffer.indexOf('\u03a3', -4, 'ucs2'), 'Second Sigma');
+
 ### buf.includes(value[, byteOffset][, encoding])
 
 * `value` String, Buffer or Number