Update the session context info

[tylerbutler]

Session context won't be passed on every check file, so we need to clarify when it would be passed and add info on scenarios that it is meant to solve (and those it shouldn't be used for).

Might also need to call out what Check properties are only used on initial session initialization.

The value passed on the query string is also Base64 encoded when it's passed back to the host in the request header.