Clarify why there's unusual sorting

Signed-off-by: Daniel Widdis <[email protected]>

modules/search-pipeline-common/src/yamlRestTest/resources/rest-api-spec/test/search_pipeline/80_sort_response.yml

@@ -147,6 +147,6 @@ teardown:
           "_source": false,
           "docvalue_fields": [ "a" ]
         }
-  # a is stored sorted for some unknown (to the test author) reason
-  # which makes it really hard to write "expected" values on tests
+  # a is stored sorted because docvalue_fields is pre-sorted to optimize aggregations
+  # this is poorly documented which makes it really hard to write "expected" values on tests
   - match: { hits.hits.0.fields: { "a": [1, 3, 4], "b": [4, 3, 1] } }