i18n documentation for required belongs_to message

When you don't explicitly provide `optional: true` to a `belongs_to`
definition, rails adds a validation to ensure presence of the
`belongs_to` validation.

However, when the validation is added, it is added with a message key of
`:required`, which was not documented in the "Translations for Active
Record Models" section of the i18n guides.

Here are the lines that add the `:required` message:
https://github.com/rails/rails/blob/c3e3577f9d5058382504773bf0d32afa15cb131e/activerecord/lib/active_record/associations/builder/belongs_to.rb#L136-L138

This commit adds the "non-optional association" validation to the table,
so rails users know how to override the message for this validation. The
default message is "must exist".

guides/source/i18n.md

@@ -888,6 +888,7 @@ So, for example, instead of the default error message `"cannot be blank"` you co
 | inclusion    | -                         | :inclusion                | -             |
 | exclusion    | -                         | :exclusion                | -             |
 | associated   | -                         | :invalid                  | -             |
+| non-optional association | -             | :required                 | -             |
 | numericality | -                         | :not_a_number             | -             |
 | numericality | :greater_than             | :greater_than             | count         |
 | numericality | :greater_than_or_equal_to | :greater_than_or_equal_to | count         |