Added :nodoc: for `attribute_changed?` and `attribute_was` [ci skip]

These methods were made "public" in 47617ecd so that `method_missing`
can invoke them without going through `send`, but they aren't meant
for consumption from outside of Rails.
Merge pull request #13300 from chancancode/add_nodoc_am_dirty_methods

Added :nodoc: for `attribute_changed?` and `attribute_was` [ci skip]