doc: add info about serializable types

querystring.stringify() doesn't serialize some values.
Explicitly mention what values are serialized in the docs.
Add what happens when another data type is passed to
`querystring.stringify()` besides the ones that can be correctly parsed

PR-URL: https://github.com/nodejs/node/pull/12313


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>