Update boxplot usage doc (#5141)

Author: stevejgordon

docs/aggregations/metric/boxplot/boxplot-aggregation-usage.asciidoc

@@ -82,6 +82,8 @@ boxplot.Max.Should().BeGreaterOrEqualTo(0);
 boxplot.Q1.Should().BeGreaterOrEqualTo(0);
 boxplot.Q2.Should().BeGreaterOrEqualTo(0);
 boxplot.Q3.Should().BeGreaterOrEqualTo(0);
+boxplot.Lower.Should().BeGreaterOrEqualTo(0);
+boxplot.Upper.Should().BeGreaterOrEqualTo(0);
 boxplot.Meta.Should().NotBeNull().And.HaveCount(1);
 boxplot.Meta["foo"].Should().Be("bar");
 ----