The serverStatus metric metrics.documents.updated is described as:

"The total number of documents updated."

This does not make a distinction between the documents which are matched by an update operations vs. the number of operations modified by the operation.

The metric reports the former - the number of operations which are matched by the operation. While this definition is correct, we should clarify that it is not the number of documents modified.

For example: "The total number of documents updated. This value is not necessarily the same as the number of documents modified by updates."