Replace "iff" with "if and only if"

iff means if and only if, but readers without that knowledge might
assume this to be a spelling mistake. To me, this seems like
exclusionary language that is unnecessary. Simply using "if and only if"
instead should suffice.

lib/net/ftp.rb

@@ -1356,7 +1356,7 @@ def close
     end
 
     #
-    # Returns +true+ iff the connection is closed.
+    # Returns +true+ if and only if the connection is closed.
     #
     def closed?
       @sock == nil or @sock.closed?