Owncloud migration documentation: Add hints to edit version.php, and to trigger background migrations after each upgrade step

The Owncloud migration documentation is helpful and mostly works, but it is missing two crucial hints:

1. The migration documentation advertises compatibility with Owncloud versions 10.13.x, 10.14.x, and 10.15.x, but only works out of the box with Owncloud version 10.13.x.
   The reason is that Nextcloud 25.0.13 declares itself compatible with Owncloud 10.13.x only in version.php. Changing this file allows the migration to proceed.
   As it's impossible to file bugs against this unsupported old Nextcloud version, the migration documentation should explain how to change the version.php file.

2. As documented in How to upgrade, we must wait for all background migrations to complete before taking the next upgrade step.
   The Owncloud migration documentation neither advises to pause and wait for background migrations to happen (or to trigger them manually), nor does it link to the latter page that explains the matter. Thus, it's easy to miss when going through the 25 → 26 → 27 → 28 → 29 → 30 → 31 upgrades. This is in fact what happened to me during a recent upgrade: Several background migrations were missed or aborted, leading to an inconsistent database schema.
   I suggest adding an explicit hint to the migration documentation.

Start with the Owncloud migration documentation and the linked How to upgrade page. Update the migration guidance to explain the version.php compatibility adjustment for Owncloud 10.14.x and 10.15.x, and to wait for or manually trigger background migrations after each upgrade step; link to the existing upgrade guidance so the sequence is unambiguous.