[1.7.x] Fixed #22710 -- Amended aggregation note for empty QuerySets

Thanks smeaton for report and Joël Rochat for spell check

Backport of 7f27cca5c5 from master

docs/ref/models/querysets.txt

@@ -2661,9 +2661,10 @@ aggregate functions, see
 
 .. admonition:: Note
 
-    Aggregation functions will return ``None`` when used with an empty
-    ``QuerySet``. For example, the ``Count`` aggregation function will return
-    ``None`` instead of ``0`` if the ``QuerySet`` contains no entries.
+    Aggregation functions return ``None`` when used with an empty
+    ``QuerySet``. For example, the ``Sum`` aggregation function returns ``None``
+    instead of ``0`` if the ``QuerySet`` contains no entries. An exception is
+    ``Count``, which does return ``0`` if the ``QuerySet`` is empty.
 
 Avg
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