Clarify batchSize option for change streams

The batchSize option for change streams is not identical to the aggregate option, so they should not share the same text. That said, the batchSize option likely has no effect on the resulting change stream (see: PHPLIB-321).

docs/includes/apiargs-MongoDBCollection-method-watch-option.yaml

-source:
-  file: apiargs-MongoDBCollection-method-aggregate-option.yaml
-  ref: batchSize
+---
+arg_name: option
+name: batchSize
+type: integer
+description: |
+  Specifies the maximum number of change events to return in each batch of the
+  response from the MongoDB cluster.
+interface: phpmethod
+operation: ~
+optional: true
 ---
 source:
   file: apiargs-MongoDBCollection-common-option.yaml