Warn about the potential to jack up /etc/passwd

This needs to be underlined. It's too easy to wang up /etc/passwd by
accident.

This closes PR rapid7#632
[Fixes #38593685]

Author: todb

modules/post/multi/escalate/metasploit_pcaplog.rb

@@ -33,7 +33,9 @@ def initialize(info={})
 
 					Successful exploitation results in the creation of a new superuser account.
 
-					This module requires manual clean-up - remove /tmp/msf3-session*pcap files and truncate /etc/passwd.
+					This module requires manual clean-up. Upon success, you should remove /tmp/msf3-session*pcap
+					files and truncate /etc/passwd. Note that if this module fails, you can potentially induce
+					a permanent DoS on the target by corrupting the /etc/passwd file.
 				},
 				'License'       => MSF_LICENSE,
 				'Author'	=> [ '0a29406d9794e4f9b30b3c5d6702c708'],