Enhance offline-first documentation on synchronization

[RushabhMendix]

Added details about synchronization modes and Compatibility Mode for entities.

As a part of this improvement- https://mendix.atlassian.net/browse/OFF-816 we made following changes - We introduced a stricter requirements check on the server synchronization API's for offline apps. This prevents entities from synchronizing changes when they are configured with the synchronization modes Online, Nothing, or Never. To allow older offline clients having an All Objects or By XPath synchronization mode to still synchronize while the new app version has the entity configured with an Online, Nothing, or Never synchronization mode, we added a Compatibility mode checkbox.

We are aiming to add following details in this document :

• Accessing objects via sync apis is no longer possible for the sync modes online / nothing / never which makes applications more secure (unless the checkbox is checked)

• Explain that the checkbox should be checked to migrate from all objects / by xpath to nothing / never / online to make sure old clients can still work

• Once all old clients are updated, the checkbox should be disabled.

Please if you feel any better place for this details.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}