document tmpname.dir

inch ci doesn't properly handle the module as a whole being marked private
and makes a stink about the documentation. no bad idea to have good docs
for internal api anyway, so just document it

lib/aptly/tmpname.rb

@@ -18,6 +18,8 @@
 module Aptly
   # Helper to generate temporary names
   module TmpName
+    # Generate a random temporary directory name.
+    # @param prefix [String] arbitrary prefix string to start the name with
     # @return [String] temporary directory name (only safe characters)
     def self.dir(prefix)
       format('%<prefix>s-%<time>s-%<pid>s-%<tid>s-%<rand>s',