Document that async Works must be empty for a 10.3 upgrade

NXP-25716 refactored the internals of fulltext-related Work classes for 10.3. In particular, FulltextUpdaterWork doesn't exist anymore and FulltextExtractorWork internals have changed. A consequence is that if one upgrades to 10.3 while still having some of these old classes instances serialized in the WorkManager, one will get errors (see NXP-26618 for instance).

So the 10.3 migration documentation should stress that one must wait for all background Work (or at least all fulltext-related ones) to be finished before doing an upgrade.