docs: added a percolator migration note
==== Deprecated percolator and mpercolate apis have been removed
Instead the `percolate` query should be used via either the search or msearch apis.
+
+==== The percolator.map_unmapped_fields_as_string setting has been deprecated
+
+The `percolator.map_unmapped_fields_as_text` setting should be used instead.
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