Add docs about nextRuntime for custom webpack

This variable is exported by next but undocumented, and its usage is necessary to build auxiliary scripts for your server-side applications. See vercel#36237 (comment) for an example.

docs/api-reference/next.config.js/custom-webpack-config.md

@@ -36,6 +36,7 @@ The second argument to the `webpack` function is an object with the following pr
 - `buildId`: `String` - The build id, used as a unique identifier between builds
 - `dev`: `Boolean` - Indicates if the compilation will be done in development
 - `isServer`: `Boolean` - It's `true` for server-side compilation, and `false` for client-side compilation
+- `nextRuntime`: `String` - The target runtime for server-side compilation; either `"edge"` or `"nodejs"`
 - `defaultLoaders`: `Object` - Default loaders used internally by Next.js:
   - `babel`: `Object` - Default `babel-loader` configuration