Fix count() docs around including null values

The count aggregate was documented to count null values, but it does not
do that. The implemented behavior is correct, so let's fix docs.

docs/source/user-guide/sql/aggregate_functions.md

@@ -123,11 +123,9 @@ bool_or(expression)
 
 ### `count`
 
-Returns the number of rows in the specified column.
+Returns the number of non-null values in the specified column.
 
-Count includes _null_ values in the total count.
-To exclude _null_ values from the total count, include `<column> IS NOT NULL`
-in the `WHERE` clause.
+To include _null_ values in the total count, use `count(*)`.
 
 ```
 count(expression)