Make it explicite that "Get All Cookies" and "Get Named Cookie" should not return an expiry key for session cookies

[whimboo]

Serializing a cookie for both the "Get All Cookies" and "Get Named Cookie" commands doesn't explicitly mention that the expiry key should not be present for session cookies:
https://w3c.github.io/webdriver/webdriver-spec.html#dfn-serialized-cookie

Only "Add Cookie" makes it clear under step 7 that for session cookies no expiry key has to be set:
https://w3c.github.io/webdriver/webdriver-spec.html#add-cookie
https://w3c.github.io/webdriver/webdriver-spec.html#dfn-cookie-expiry-time