Add information about limitations and upgrade of nuxeo.io environments

Update the page http://doc.nuxeo.com/x/hgcuAQ with a limitation section that explains the following:

• emails are not configured
• No System info > Configuration tab
• No Update Center > Local Packages tab
• Instance is correctly registered
• It's not possible to unregister from a nuxeo.io instance
• Update Center > Studio: Dev mode activated
• Elasticsearch: only the Admin tab

Add an upgrade section that explains how to an environment is upgraded (upgrade target version of associated studio project or change studio project for a more recent version)