Docs: document hard_bounds parameter in histograms #59774
Description
In #59175 we added support for hard_bounds to histogram and date_histogram, which is a counterpart to the existing "extended_bounds" parameter, which would push out the boundaries of the histogram even if there wasn't data to fill the new bounds. It is needed to support unbounded ranges, which otherwise forces histogram to keep creating new buckets until the end of time since the range is unbounded. Users can now specify the absolute max extents of their histogram and we won't create any further buckets. This feature was added to 7.10.0 and not currently documented.