Add missing footer notes in mapper size docs

docs/plugins/mapper-size.asciidoc

@@ -103,3 +103,7 @@ GET my_index/_search
 // TEST[continued]
 
 <1> Querying on the `_size` field
+<2> Aggregating on the `_size` field
+<3> Sorting on the `_size` field
+<4> Accessing the `_size` field in scripts (inline scripts must be modules-security-scripting.html#enable-dynamic-scripting[enabled] for this example to work)
+