Add documentaiton to the new labeling functionality

We add documentation on how this could be used

lib/kino/process.ex

@@ -225,6 +225,14 @@ defmodule Kino.Process do
   target argument can either be a single PID, a list of PIDs, or the atom `:all`
   depending on what messages you would like to retain in your trace.
 
+  ## Options
+
+    * `:label_function` - A function to help label message events. If
+      the given function returns `:continue`, then the default label
+      is used. However, if the function returns a `String.t()`, then
+      that will be used for the label. To use the default labeling
+      logic in the given function see `label_from_value/1`.
+
   ## Examples
 
   To generate a trace of all the messages occurring during the execution of the
@@ -443,6 +451,12 @@ defmodule Kino.Process do
   defp label_from_reason({:shutdown, _}), do: "shutdown"
   defp label_from_reason(_), do: "abnormal"
 
+  @doc """
+  Generates a label given a value.
+
+  This function is used in the labeling process to label values, after
+  the normal prefix string (CALL:, CAST:, etc.).
+  """
   def label_from_value(tuple)
       when is_tuple(tuple) and is_atom(elem(tuple, 0)),
       do: elem(tuple, 0)