(maint) Fix ctrl::do_until docs

The docs mischaracterized the return value of ctrl::do_until as `nil`.
That function returns the value of the given code block's last iteration

bolt-modules/ctrl/lib/puppet/functions/ctrl/do_until.rb

@@ -6,7 +6,7 @@
   # @param block The code block to repeat.
   # @option options [Numeric] limit The number of times to repeat the block.
   # @option options [Numeric] interval The number of seconds to wait before repeating the block.
-  # @return [nil]
+  # @return The value of the code block's last iteration
   # @example Run a task until it succeeds
   #   ctrl::do_until() || {
   #     run_task('test', $target, '_catch_errors' => true).ok()