Install Docu: /etc/hosts declaration for FQDN

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==== Hello,

why is it worth for a "Warning" notice inside the ubuntu install ducumentation?

    /etc/hosts: hostname <=> IP address mapping. Warning: List the FQDN hostname as first item.

# Part of file: /etc/hosts
127.0.0.1   mx.example.com demo localhost localhost.localdomain

Which service get in trouble if it looks like:

127.0.0.1  demo localhost localhost.localdomain  mx.example.com

BR,