Document ArrayBuffer support in k6/encoding module

Part of #233

Author: Ivan Mirić

src/data/markdown/docs/02 javascript api/05 k6-encoding/b64encode- input- -encoding- -.md

@@ -1,18 +1,18 @@
 ---
 title: 'b64encode( input, [encoding] )'
-description: 'Base64 encode a string.'
+description: 'Encode data in base64.'
 ---
 
-| Parameter           | Type   | Description                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
-| ------------------- | ------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
-| input               | string | The string to base64 encode.                                                                                                                                                                                                                                                                                                                                                                                                                                              |
+| Parameter           | Type   | Description                                                              |
+| ------------------- | ------ | ------------------------------------------------------------------------ |
+| input               | string / ArrayBuffer | The input string or `ArrayBuffer` object to base64 encode. |
 | encoding (optional) | string | The base64 encoding to use.<br/>Available options are:<br/>- **"std"**: the standard encoding with `=` padding chars and `+` and `/` characters in encoding alphabet. This is the default.<br/>- **"rawstd"**: like `std` but without `=` padding characters.<br/>- **"url"**: URL safe version of `std`, encoding alphabet doesn't contain `+` and `/` characters, but rather `-` and `_` characters.<br/>- **"rawurl"**: like `url` but without `=` padding characters. |
 
 ### Returns
 
-| Type   | Description                                       |
-| ------ | ------------------------------------------------- |
-| string | The base64 encoded version of the `input` string. |
+| Type   | Description                              |
+| ------ | ---------------------------------------- |
+| string | The base64 encoding of the `input` data. |
 
 ### Example
 
@@ -25,9 +25,11 @@ import encoding from 'k6/encoding';
 export default function () {
   let str = 'hello world';
   let enc = 'aGVsbG8gd29ybGQ=';
+  let buf = new Uint8Array([104, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100]).buffer;
   check(null, {
     'is encoding correct': () => encoding.b64encode(str) === enc,
     'is decoding correct': () => encoding.b64decode(enc) === str,
+    'is encoding ArrayBuffer correct': () => encoding.b64encode(buf) === enc,
   });
 }
 ```