We see questions about the output of the change streams.
When user issues update command it may result in change type:

• 'update'
• 'replace', if writing down the diff is more verbose than the final version of the document
• 'insert', if upset:true was set and no matching documents wre found to update

For update there's sub-case for how updates to arrays are represented. Here is example of the question/complaint SERVER-75590. Similarly to the whole-document update vs insert, we may output truncation of array, update to the individual array element or rewrite entire array. We are biased toward for concise representation of the updates where possible (although some legacy update operators might still produce more lengthy versions)

I'm not sure what is the best place for it in docs, but I'd like us to add some info around it. The intent is not to document the minutia of the current logic (we want to keep room for potential changes), but to highlight that when building change processors, users should be ready to receive descriptions of the updates in the various formats as it's hard to predict how exactly each operation will be represented in the change event.