docs(config): update hashFunction docs based on v4 (#1761)

src/content/configuration/output.md

@@ -160,7 +160,7 @@ If multiple modules would result in the same name, [`output.devtoolFallbackModul
 
 `string`
 
-This option determines the modules namespace used with the [`output.devtoolModuleFilenameTemplate`](#output-devtoolmodulefilenametemplate). When not specified, it will default to the value of: [`output.library`](#output-library). It's use is to prevent source file path collisions in sourcemaps when loading multiple libraries built with webpack.
+This option determines the modules namespace used with the [`output.devtoolModuleFilenameTemplate`](#output-devtoolmodulefilenametemplate). When not specified, it will default to the value of: [`output.library`](#output-library). It's used to prevent source file path collisions in sourcemaps when loading multiple libraries built with webpack.
 
 For example, if you have 2 libraries, with namespaces `library1` and `library2`, which both have a file `./src/index.js` (with potentially different contents), they will expose these files as `webpack://library1/./src/index.js` and `webpack://library2/./src/index.js`.
 
@@ -236,8 +236,15 @@ The prefix length of the hash digest to use, defaults to `20`.
 
 ## `output.hashFunction`
 
-The hashing algorithm to use, defaults to `'md5'`. All functions from Node.JS' [`crypto.createHash`](https://nodejs.org/api/crypto.html#crypto_crypto_createhash_algorithm) are supported.
+`string|function`
 
+The hashing algorithm to use, defaults to `'md5'`. All functions from Node.JS' [`crypto.createHash`](https://nodejs.org/api/crypto.html#crypto_crypto_createhash_algorithm) are supported. Since `4.0.0-alpha2`, the `hashFunction` can now be a constructor to a custom hash function. You can provide a non-crypto hash function for performance reasons.
+
+``` js
+hashFunction: require('metrohash').MetroHash64
+```
+
+Make sure that the hashing function will have `update` and `digest` methods available.
 
 ## `output.hashSalt`