Update guide for validation custom contexts [ci skip]
Update guide for validation custom contexts [ci skip]

The Active Record validations guide's section on custom contexts appears
to be incomplete. the code sample shows a context being added to
validations, but not being used.

Add to the sample code for this section by showing validations being run
with and without the custom context. Add a second sample code block
showing that validations with no context are also run, when a context is
used.

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