Book: explain statically checked unreachable

[SuperFluffy]

The current version of the Handling Disconnects section of the book states:

In the shutdown case we use match void {} as a statically-checked unreachable!().

Please explain the significance of this statement. In how far is this a statically checked version of unreachable!?

This is the first time I am encountering this pattern and I am confused by the statement.

[skade]

cc @matklad

[tomasol]

I also struggled with that section, reading about Empty types helped me.