clarify doc of NoHostAuthenticationForLocalhost

Clarify the documentation of the NoHostAuthenticationForLocalhost directive in
ssh_config(5):

• Document, that it works on any hostname that resolves to the loopback device.
• Demote the „use case” to being just one example of how it can be used.

Fixes bug #2293.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>