MMS OnPrem: Document optional 2FA setup

• It will only appear as an option for an individual user to set up under Settings if the OnPrem installation has been set up with a full Twilio integration. (i.e. they filled in all the Twilio information in their conf-mms.properties file.)
• If this has been done, then the option to set up 2FA for an individual user account will show up under Settings
• If the user optionally sets up 2FA they will then need to use 2FA for certain backup actions, and to login if they are a global administrator
• If the individual user no longer wishes to use 2FA, then a global administrator can remove 2FA from their account using the Edit functionality at Admin : Users

Please backport to MMS 1.4