Add some documentation to Timeout#timeout about possible issues

Documentation requested in [Bug #15886].

lib/timeout.rb

@@ -67,7 +67,9 @@ module Timeout
   # +sec+ seconds, otherwise throws an exception, based on the value of +klass+.
   #
   # The exception thrown to terminate the given block cannot be rescued inside
-  # the block unless +klass+ is given explicitly.
+  # the block unless +klass+ is given explicitly. However, the block can use
+  # ensure to prevent the handling of the exception.  For that reason, this
+  # method cannot be relied on to enforce timeouts for untrusted blocks.
   #
   # Note that this is both a method of module Timeout, so you can <tt>include
   # Timeout</tt> into your classes so they have a #timeout method, as well as