Adds setting level to allocation decider explanations (#22268)

The allocation decider explanation messages where improved in #21771 to
include the specific Elasticsearch setting that contributed to the
decision taken by the decider.  This commit improves upon the
explanation message output by including whether the setting was an index
level setting or a cluster level setting.  This will further help the
user understand and locate the setting that is the cause of shards
remaining unassigned or remaining on their current node.

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