Clarify that index.batchThreads is relevant for offline reindexing
The documentation did not say that the setting would not affect offline
reindexing. But by putting focus on the effect on online reindexing, it
somewhat suggested it. That would be misleading and might lead people
to run offline reindexing with index.batchThreads a conservative 1 from
a production environment and consequently leave most CPUs unnecessarily
idling for offline reindexing.