Today we indicate that the `unassigned.reason` field in various APIs
indicates the reason why a shard is unassigned. This isn't really true,
it tells you some information about the event that caused the shard to
_become_ unassigned (or which most recently changed its routing table
entry while remaining unassigned) but tells you almost nothing about why
the shard _is now_ unassigned and how to fix it. That's what the
allocation explain API is for. This commit clarifies this point in the
docs.

Closes elastic#80892

Co-authored-by: James Rodewig <james.rodewig@elastic.co>