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>