Migration to Icedove: Users don't realise how to stop the warning

Originally created by @emmapeel on #11187 (Redmine)

Users don’t understand that they need to disable persistence of Claws Mail after migration, and they keep clicking on the warning ‘You need to migrate to Icedove’.

The warning (or the migration howto) should mention this step, as some users are still clicking every time they start Icedove.

Feature Branch: doc/11187-claws-mail-persistence