Add notes regarding closing sessions when working with streams (#252)

lib/mongo.ex

@@ -418,7 +418,19 @@ defmodule Mongo do
   end
 
   @doc """
-  Performs aggregation operation using the aggregation pipeline.
+  Performs aggregation operation using the aggregation pipeline and returns a Mongo.Stream.
+  It should be noted that code that uses the paginated query results without engaging Mongo.Streams Enumerable behavior
+  can result in the sessions hanging around and causing resource exhaustion.
+
+    Example:
+      # Results in an open session
+      %Mongo.Stream{docs: docs} = Mongo.aggregate(@topology, collection, pipeline, opts)
+      docs |> Enum.map(fn elem -> elem end)
+
+      # Results in a closed session via the Enumerable protocol
+      Mongo.aggregate(@topology, collection, pipeline, opts)
+      |> Enum.map(fn elem -> elem end)
+
 
   For all options see [Options](https://docs.mongodb.com/manual/reference/command/aggregate/#aggregate)
 

lib/mongo/session.ex

@@ -62,6 +62,8 @@ defmodule Mongo.Session do
 
   For more information about causal consistency see the [officially documentation](https://docs.mongodb.com/manual/core/read-isolation-consistency-recency/#causal-consistency).
 
+  Note that Mongo.Stream implements the Enumerable protocol and the reduce/3 function calls Mongo.Stream.checkin_session/3 after the stream is exhausted.
+
   If you want to use transaction, then you need to create a session as well:
 
       alias Mongo.Session

lib/mongo/stream.ex

@@ -1,5 +1,8 @@
 defmodule Mongo.Stream do
-  @moduledoc false
+  @moduledoc """
+    This module provides a stream implementation that automatically checks out a session when the stream is started and an Enumerable
+    protocol that checks it back in when the stream has been consumed.
+  """
 
   alias Mongo.Session
   alias Mongo.Error