Mention block form of Struct.new in Style/StructInheritance

[XrXr]

As the documentation for Struct recommends using the block
form over inheriting, I think it is worth mentioning it.

If a block is given it will be evaluated in the context of
StructClass, passing the created class as a parameter

This is the recommended way to customize a struct.

Adjust examples in the documentation and add a test which uses
the block form.

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[bbatsov]

Thanks! Probably it's worth extending the rationale of the cop as well.