Use :nodoc: for alias_method instead of :stopdoc: [ci-skip]

RDoc 6.7 now hides `alias_method`s marked with `:nodoc:`.

activesupport/lib/active_support/values/time_zone.rb

@@ -208,9 +208,7 @@ def find_tzinfo(name)
         TZInfo::Timezone.get(MAPPING[name] || name)
       end
 
-      # :stopdoc:
-      alias_method :create, :new
-      # :startdoc:
+      alias_method :create, :new # :nodoc:
 
       # Returns a TimeZone instance with the given name, or +nil+ if no
       # such TimeZone instance exists. (This exists to support the use of