Update date histogram doc (#5108)

Author: stevejgordon

docs/aggregations/bucket/date-histogram/date-histogram-aggregation-usage.asciidoc

@@ -19,7 +19,7 @@ A multi-bucket aggregation similar to the histogram except it can only be applie
 From a functionality perspective, this histogram supports the same features as the normal histogram.
 The main difference is that the interval can be specified by date/time expressions.
 
-NOTE: When specifying a `format` **and** `extended_bounds` or `missing`, in order for Elasticsearch to be able to parse
+NOTE: When specifying a `format` **and** `extended_bounds`, `hard_bounds` or `missing`, in order for Elasticsearch to be able to parse
 the serialized `DateTime` of `extended_bounds` or `missing` correctly, the `date_optional_time` format is included
 as part of the `format` value.