Add info on setting polymorphic serializers (#1906)

While this same document provides details on how to override the
serializer_for, not all users will realize this could be used to set the
sterilizers for polymorphic relationships.  This change just adds a link
to that documentation and makes that point obvious.

Author: Rio517

docs/general/serializers.md

@@ -140,7 +140,7 @@ class PictureSerializer < ActiveModel::Serializer
 end
 ```
 
-For more context, see the [tests](../../test/adapter/polymorphic_test.rb) for each adapter.
+You can specify the serializers by [overriding serializer_for](serializers.md#overriding-association-serializer-lookup). For more context about polymorphic relationships, see the [tests](../../test/adapter/polymorphic_test.rb) for each adapter.
 
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