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: nodejs/node#12313
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/querystring.md

@@ -91,6 +91,10 @@ added: v0.1.25
 The `querystring.stringify()` method produces a URL query string from a
 given `obj` by iterating through the object's "own properties".
 
+It serializes the following types of values passed in `obj`:
+{string|number|boolean|string[]|number[]|boolean[]}
+Any other input values will be coerced to empty strings.
+
 For example:
 
 ```js