Document the default value for zk_session_timeout in Marathon 0.9 - 0.13.
Document the default value for zk_session_timeout in Marathon 0.9 - 0.13.

Summary:
The default value has been changed for different versions of Marathon.

Depends: D181

Test Plan: reading

Reviewers: jmlvanre

Reviewed By: jmlvanre

Subscribers: jenkins, marathon-team

JIRA Issues: DCOS-9930

Differential Revision: https://phabricator.mesosphere.com/D182

guenter/marathon,natemurthy/marathon,mesosphere/marathon,mesosphere/marathon,gsantovena/marathon,guenter/marathon,janisz/marathon,Caerostris/marathon,Caerostris/marathon,meln1k/marathon,meln1k/marathon,natemurthy/marathon,janisz/marathon,gsantovena/marathon,gsantovena/marathon,mesosphere/marathon,janisz/marathon,meln1k/marathon,mesosphere/marathon,Caerostris/marathon,Caerostris/marathon,mesosphere/marathon,meln1k/marathon,meln1k/marathon,gsantovena/marathon,janisz/marathon,Caerostris/marathon,natemurthy/marathon,guenter/marathon,gsantovena/marathon,guenter/marathon,janisz/marathon,guenter/marathon