Update documentation and deprecation message

actionpack/lib/action_dispatch/middleware/ssl.rb

@@ -23,7 +23,7 @@ module ActionDispatch
   #     preload lists is `18.weeks`.
   #   * `subdomains`: Set to `true` to tell the browser to apply these settings
   #     to all subdomains. This protects your cookies from interception by a
-  #     vulnerable site on a subdomain. Defaults to `false`.
+  #     vulnerable site on a subdomain. Defaults to `true`.
   #   * `preload`: Advertise that this site may be included in browsers'
   #     preloaded HSTS lists. HSTS protects your site on every visit *except the
   #     first visit* since it hasn't seen your HSTS header yet. To close this
@@ -62,8 +62,8 @@ def initialize(app, redirect: {}, hsts: {}, secure_cookies: true, **options)
         hsts[:subdomains] = false
 
         ActiveSupport::Deprecation.warn <<-end_warning.strip_heredoc
-          In Rails 5.1, HSTS support for subdomains will be turned on by default.
-          Set `config.ssl_options = { hsts: { subdomains: false }}` to opt out
+          In Rails 5.1, The `:subdomains` option of HSTS config will be treated as true if
+          unspecified. Set `config.ssl_options = { hsts: { subdomains: false }}` to opt out
           of this behavior.
         end_warning
       end