Docs: Clarify that es6 env also sets ecmaVersion to 6 (#7067)

Reading the docs, it is not clear why ESLint's parser will stop complaining about ES6 specifics when `es6` env is set and `ecmaVersion: 6` is omitted.
It is actually done [on purpose](https://github.com/eslint/eslint/blob/e1187285f137d685bcfc9e2a771fa059ef9637a9/conf/environments.js#L98-L103), but the documentation didn't mention this at all.

docs/user-guide/configuring.md

@@ -84,7 +84,7 @@ An environment defines global variables that are predefined. The available envir
 * `node` - Node.js global variables and Node.js scoping.
 * `commonjs` - CommonJS global variables and CommonJS scoping (use this for browser-only code that uses Browserify/WebPack).
 * `shared-node-browser` - Globals common to both Node and Browser.
-* `es6` - enable all ECMAScript 6 features except for modules.
+* `es6` - enable all ECMAScript 6 features except for modules (this automatically sets the `ecmaVersion` parser option to 6).
 * `worker` - web workers global variables.
 * `amd` - defines `require()` and `define()` as global variables as per the [amd](https://github.com/amdjs/amdjs-api/wiki/AMD) spec.
 * `mocha` - adds all of the Mocha testing global variables.