Clarify when association needs to be preloaded for cast/put_assoc

lib/ecto/changeset.ex

@@ -522,9 +522,10 @@ defmodule Ecto.Changeset do
   @doc """
   Casts the given association with the changeset parameters.
 
-  This function requires the association to have been
+  When updating the data, this function requires the association to have been
   preloaded in the changeset struct. Missing data will
   invoke the `:on_replace` behaviour defined on the association.
+  Preloading is not necessary for newly built structs.
 
   The parameters for the given association will be retrieved
   from `changeset.params`. Those parameters are expected to be
@@ -990,9 +991,10 @@ defmodule Ecto.Changeset do
   @doc """
   Puts the given association as a change in the changeset.
 
-  This function requires the association to have been
+  When updating the data, this function requires the association to have been
   preloaded in the changeset struct. Missing data will
   invoke the `:on_replace` behaviour defined on the association.
+  Preloading is not necessary for newly built structs.
 
   The given value may either be the association struct, a
   changeset for the given association or a map or keyword