clarify how IgnoreUserKnownHosts works

Based on the previous documentation of the IgnoreUserKnownHosts directive, the
average user could easily think that the default value “no” is the more secure
choice (in the sense of “do not even check in ~/.ssh/known_hosts”).

• Clarify in sshd_config(5), that a value of “yes” in the IgnoreUserKnownHosts
  directive, makes sshd(8) only trust the global known hosts list (/etc/ssh/
  ssh_known_hosts).

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>