* (core) - Add staleWhileRevalidate to ssrExchange

* Add staleWhileRevalidate to next-urql

* (docs) - document ssrExchange/next-urql staleWhileRevalidate option (#1853)

* add docs about staleWhileRevalidate

* Apply suggestions from code review
Co-authored-by: Phil Pluckthun <phil@kitten.sh>

* Apply suggestions from code review
Co-authored-by: Phil Pluckthun <phil@kitten.sh>
Co-authored-by: Phil Pluckthun <phil@kitten.sh>
Co-authored-by: Jovi De Croock <jovi.decroock@formidable.com>