Remove useless comment and fix yard docs

lib/mutant/cache.rb

@@ -1,7 +1,6 @@
 module Mutant
   # An AST cache
   class Cache
-    # This is explicitly empty! Ask me if you are interested in reasons :D
     include Equalizer.new
 
     # Initialize object
@@ -16,6 +15,8 @@ module Mutant
 
     # Return node for file
     #
+    # @param [#to_s] path
+    #
     # @return [AST::Node]
     #
     # @api private