Update ms17_010_eternalblue description and ranking.

The module has been noted to cause crashes, reboots, BSOD, etc, on
some systems.

modules/exploits/windows/smb/ms17_010_eternalblue.rb

@@ -8,7 +8,7 @@ require 'ruby_smb/smb1/packet'
 require 'windows_error'
 
 class MetasploitModule < Msf::Exploit::Remote
-  Rank = GoodRanking
+  Rank = AverageRanking
 
   include Msf::Exploit::Remote::Tcp
 
@@ -32,6 +32,9 @@ class MetasploitModule < Msf::Exploit::Remote
         The module will attempt to use Anonymous login, by default, to authenticate to perform the
         exploit. If the user supplies credentials in the SMBUser,SMBPass, and SMBDomain options it will use
         those instead.
+
+        On some systems, this module may cause system instability and crashes, such as a BSOD or
+        a reboot. This may be more likely with some payloads.
       },
 
       'Author'         => [