Clarify new_framework_defaults instructions

Framework defaults cause confusion for contributors when adding new
defaults. Should the defaults be defined in:
`railties/lib/rails/application/configuration.rb` and/or
`config/initializers/new_framework_defaults_7_0.rb.tt`
And should these define the new or old defaults?

Hopefully this clarifies it by making it clear the
new framework defaults are commented out.
By mentioning `config.load_defaults` it hopefully clarifies that these
defaults should be similar to the onces defined by `load_defaults`.

Also reword "flip" to "uncomment" to make it clear you should uncomment
the config instead of setting `true` to `false`.

Author: p8

railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_7_0.rb.tt

@@ -1,8 +1,10 @@
 # Be sure to restart your server when you modify this file.
 #
-# This file contains migration options to ease your Rails 7.0 upgrade.
+# This file eases your Rails 7.0 framework defaults upgrade.
 #
-# Once upgraded flip defaults one by one to migrate to the new default.
+# Uncomment each configuration one by one to switch to the new default.
+# Once your application is ready to run with all new defaults, you can remove
+# this file and set the `config.load_defaults` to `7.0`.
 #
 # Read the Guide for Upgrading Ruby on Rails for more info on each option.