doc: improve description of urlObject.query

The description of urlObject.query is ambiguous about when it's an
object vs when it's a string. Added a sentence pointing to the option
that determines this in url.parse().

Also fixed the missing parentheses in the first sentence by rewording it to
avoid nested parentheses.

PR-URL: https://github.com/nodejs/node/pull/9625
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api/url.md

@@ -114,9 +114,10 @@ No decoding of the `path` is performed.
 
 ### urlObject.query
 
-The `query` property is either the "params" portion of the query string (
+The `query` property is either the query string without the leading ASCII
-everything *except* the leading ASCII question mark (`?`), or an object
+question mark (`?`), or an object returned by the [`querystring`][] module's
-returned by the [`querystring`][] module's `parse()` method:
+`parse()` method. Whether the `query` property is a string or object is
+determined by the `parseQueryString` argument passed to `url.parse()`.
 
 For example: `'query=string'` or `{'query': 'string'}`