Document market (app) related upgrade information

[patrickjahns]

With the improved switches provided via owncloud/core#28812 it is now possible to do upgrades with/without market app enabled.

We should document how user can disable the market app (i.e. for clustered setups) during the upgrade process.

Also off the top of my head, document the differences between the switches

1. market app disable/not available -> it will not be used during upgrade
2. 'upgrade.automatic-app-update' => false, don't do automatic app upgrades during upgrade process
3. 'appstoreenabled' => false,

More details are in the flowchart from vincent