samples: net: mdns_responder: Code in example is unrelated to mDNS

Clarify the comment in the code so people looking to use mDNS don't
think that interaction with Zephyr is necessary for mDNS support to
work, its in fact built into the networking stack when the mDNS
feature is enabled.

Signed-off-by: Chris Morgan <chmorgan@gmail.com>