Improve exception message for Jason.Encoder errors (#4534)

Author: ivanov

lib/ecto/json.ex

@@ -17,6 +17,9 @@ if Code.ensure_loaded?(Jason.Encoder) do
 
             @derive {Jason.Encoder, only: [:name, :title, ...]}
             schema ... do
+
+      You can also use the :except option instead of :only if you would \
+      prefer to skip some fields.
       """
     end
   end
@@ -29,13 +32,16 @@ if Code.ensure_loaded?(Jason.Encoder) do
       exposed externally.
 
       You can either map the schemas to remove the :__meta__ field before \
-      encoding to JSON, or explicitly list the JSON fields in your schema:
+      encoding or explicitly list the JSON fields in your schema:
 
           defmodule #{inspect(schema)} do
             # ...
 
             @derive {Jason.Encoder, only: [:name, :title, ...]}
             schema ... do
+
+      You can also use the :except option instead of :only if you would \
+      prefer to skip some fields.
       """
     end
   end