Added documentation on receive do blocks in Elixir.

elixir.html.markdown

@@ -369,6 +369,13 @@ spawn(f) #=> #PID<0.40.0>
 # messages to the process. To do message passing we use the `send` operator.
 # For all of this to be useful we need to be able to receive messages. This is
 # achieved with the `receive` mechanism:
+
+# The `receive do` block is used to listen for messages and process
+# them when they are received. A `receive do` block will only
+# process one received message. In order to process multiple
+# messages, a function with a `receive do` block must recursively
+# call itself to get into the `receive do` block again.
+
 defmodule Geometry do
   def area_loop do
     receive do