Attempt clarification of `set_missing_host_key_policy` param.
Attempt clarification of `set_missing_host_key_policy` param.

This trips up lots of users. Re #45

SebastianDeiss/paramiko,jaraco/paramiko,ameily/paramiko,dorianpula/paramiko,reaperhulk/paramiko,mirrorcoder/paramiko,paramiko/paramiko