Provide more clarity on the length of list in c:prepare_messages/2 (#…

…333)

lib/broadway.ex

@@ -814,10 +814,15 @@ defmodule Broadway do
   by `c:handle_message/3`. For example, if you need to query the database,
   instead of doing it once per message, you can do it on this callback.
 
-  The length of the list of messages received by this callback is based on
-  the `min_demand`/`max_demand` configuration in the processor. This callback
-  must always return all messages it receives, as `c:handle_message/3` is still
-  called individually for each message afterwards.
+  The length of the list of messages received by this callback is often based
+  on the `min_demand`/`max_demand` configuration in the processor but ultimately
+  it depends on the producer and the frequency data arrives. A pipeline that
+  receives one message per minute will most likely emit single element lists.
+  Producers which are push-based, rather than pull-based, such as
+  `BroadwayRabbitMQ.Producer`, are more likely to send messages as they appear.
+
+  This callback must always return all messages it receives, as
+  `c:handle_message/3` is still called individually for each message afterwards.
   """
   @callback prepare_messages(messages :: [Message.t()], context :: term) :: [Message.t()]