Better explanation of split logic.

Author: kisieland

processor/batchprocessor/README.md

@@ -17,6 +17,9 @@ Please refer to [config.go](./config.go) for the config spec.
 The following configuration options can be modified:
 - `send_batch_size` (default = 8192): Number of spans, metric data points, or log
 records in metrics after which a batch will be sent regardless of the timeout.
+While creating batches of metrics, the data is being split on metric level and
+limited by number of data points. Because of that some of the batches might end
+up smaller than this value.
 - `timeout` (default = 200ms): Time duration after which a batch will be sent
 regardless of size.
 - `send_batch_max_size` (default = 0): The upper limit of the batch size.