docs: Updates to N Preview ICU4J doc.

New CL to address Dave's comments on the first draft of the ICU4J doc.
This CL continues the work in CL http://ag/873979 ; see that CL for the
D's review comments and the responses.

See first comment for doc stage location.

bug: 27076197
Change-Id: Ie7ef2240b9bd3b35e0fc65f6264e2f4c7014980b