Added nested filter join option to the docs.

Closes #3738

docs/reference/query-dsl/filters/nested-filter.asciidoc

@@ -33,3 +33,46 @@ the `_name` value). For example:
     }
 }
 --------------------------------------------------
+
+[float]
+==== Join option
+
+The nested filter also supports an `join` option. Which controls whether to perform the block join.
+By default this enabled, but when disabled it emits the hidden nested documents as hits instead of the joined root document.
+
+This is useful when a `nested` filter is used in a facet where nested is enabled, like you can see in the example below:
+
+[source,js]
+--------------------------------------------------
+{
+    "query" : {
+		"nested" : {
+			"path" : "offers",
+			"query" : {
+				"match" : {
+					"offers.color" : "blue"
+				}
+			}
+		}
+	},
+	"facets" : {
+		"size" : {
+			"terms" : {
+				"field" : "offers.size"
+			},
+			"facet_filter" : {
+				"nested" : {
+					"path" : "offers",
+					"query" : {
+						"match" : {
+							"offers.color" : "blue"
+						}
+					},
+					"join" : false
+				}
+			},
+			"nested" : "offers"
+		}
+	}
+}'
+--------------------------------------------------