Update documentation

docs/syntax.md

@@ -39,7 +39,7 @@ A good example of a parser is [Safe Parser], which parses malformed/broken CSS.
 Because there is no point to generate broken output, this package only provides
 a parser.
 
-The parser API is a function which receives a string & returns a [`Root`] node.
+The parser API is a function which receives a string & returns a [`Root`] or [`Document`] node.
 The second argument is a function which receives an object with PostCSS options.
 
 ```js
@@ -54,6 +54,7 @@ module.exports = function parse (css, opts) {
 
 [Safe Parser]: https://github.com/postcss/postcss-safe-parser
 [`Root`]:      https://postcss.org/api/#root
+[`Document`]:  https://postcss.org/api/#document
 
 
 ### Main Theory
@@ -170,7 +171,7 @@ The Stringifier API is little bit more complicated, than the parser API.
 PostCSS generates a source map, so a stringifier can’t just return a string.
 It must link every substring with its source node.
 
-A Stringifier is a function which receives [`Root`] node and builder callback.
+A Stringifier is a function which receives [`Root`] or [`Document`] node and builder callback.
 Then it calls builder with every node’s string and node instance.
 
 ```js