Clarify ActiveStorage::Service#url docs [ci skip]

* Public services don’t respect `:disposition`, `:filename`, or `:content_type`.
* `:expires_in` is optional.

activestorage/lib/active_storage/service.rb

@@ -106,9 +106,9 @@ def exist?(key)
     end
 
     # Returns the URL for the file at the +key+. This returns a permanent URL for public files, and returns a
-    # short-lived URL for private files. You must provide the +disposition+ (+:inline+ or +:attachment+),
-    # +filename+, and +content_type+ that you wish the file to be served with on request. In addition, for
-    # private files, you must also provide the amount of seconds the URL will be valid for, specified in +expires_in+.
+    # short-lived URL for private files. For private files you can provide the +disposition+ (+:inline+ or +:attachment+),
+    # +filename+, and +content_type+ that you wish the file to be served with on request. Additionally, you can also provide
+    # the amount of seconds the URL will be valid for, specified in +expires_in+.
     def url(key, **options)
       instrument :url, key: key do |payload|
         generated_url =