Add documentation about DELETE /session

@ametaireau r?
The idea is here, but here you're really describing what the server does while I would have expected to see what is the right behavior.

Something along the lines of 

>This should be used to clear the hawk session of a Firefox Account user. You should not attempt to call this endpoint with a non-firefox account session, since it would mean as a client you could not attach a session anymore. 
> In case you want to destroy a non-FxA session, please use the DELETE /account endpoint.