fix documentation about undefined_field

Summary: - as title, the previous paragraph was wrong

Reviewed By: VLanvin

Differential Revision: D66246696

fbshipit-source-id: 440f08d37633d054f0fe158e877714407150c0ca

docs/reference/errors.md

@@ -151,7 +151,18 @@ test_neg() ->
 
 ### undefined_field
 
-This error occurs when attempting to create a record with a field that does not exist.
+This error occurs when attempting to create a record without specifying the value for a field
+when the field type doesn't allow `undefined` values.
+
+Example:
+```Erlang
+-record(user, {name :: string(), id :: binary()}).
+
+-spec create_user() -> #user{}.
+create_user() -> #user{id = <<"000">>}.
+
+% #user{...}: name is undefined
+```
 
 
 ### unbound_var