[DOCS] Fix the weighed average documentation (elastic#81307)

The documentations states that if the `weight` field is missing, and no
explicit missing configuration is provided, a default value of 1 is used.
This is incorrect and does not match the implementation of the weighted
average aggregator. In this specific case the document is skipped, instead.

docs/reference/aggregations/metrics/weighted-avg-aggregation.asciidoc

@@ -207,13 +207,8 @@ Which should look like:
 
 ==== Missing values
 
-The `missing` parameter defines how documents that are missing a value should be treated.
-The default behavior is different for `value` and `weight`:
-
-By default, if the `value` field is missing the document is ignored and the aggregation moves on to the next document.
-If the `weight` field is missing, it is assumed to have a weight of `1` (like a normal average).
-
-Both of these defaults can be overridden with the `missing` parameter:
+By default, the aggregation excludes documents with a missing or `null` value for the `value` or `weight` field. Use the
+ `missing` parameter to specify a default value for these documents instead.
 
 [source,console]
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