Fix errors for four of the code samples
The four code samples that fail to run are:

- Add attribute magic to objects. Fixed by introducing a Person
  instance variable.

- Tracking value changes. Fixed by replacing `attr_accessor` with
  `define_attribute_methods`, providing getter and setter methods
  for `name` and providing the missing `Person#save` method. A
  call to `Person#save` has to precede the `person.name = 'robert'`
  assignment, if we want `previous_changes` to include 'bob'.

- Adding `errors` interface to objects. Fixed by introducing a
  Person instance variable, assigning `nil` to its name and calling
  `Person#validate!`.

- Custom validators. Fixed by defining `HasNameValidator` before
  it is used by `ValidatorPerson`.

All the code samples can now be run smoothly.

Call Dirty#changes_applied in Person#save, instead of modifying instance vars.
Merge pull request #14313 from m-Peter/active_model_doc_fixes

Fix errors for four of the code samples [ci skip]
Move setting :scope_level_resource to resource_scope

Originally with_scope_level was exclusively for managing scope levels with
resources, however it is now used for other things so it makes more sense
to move the responsibility for setting the :scope_level_resource to the
resource_scope method. This eliminates repeatedly setting it to the same
resource as each resource method scope is evaluated.
Only use shallow nested scope when depth is > 1

By tracking the depth of resource nesting we can push the need for nested
shallow scoping to only those routes that are nested more than one deep.
This allows us to keep the fix for #12498 and fix the regression in #14224.

Fixes #14224.
Pull namespace defaults out of the options hash

If a developer has specified either :path or :as in the options hash then
these should be used as the defaults for :shallow_path and :shallow_prefix.

Fixes #14241.
Copy shallow options from normal options when using scope

If the options :shallow_prefix and :shallow_path are not set in the
scope options then copy them from the normal :as and :path options
if they are set.
Move changelog entry to the top, highlight module name [ci skip]
Disable `source_type` scope when `through` already scoped

Add a condition to disable the `source_type` automatic class scoping
when the `through_reflection` already has a scope.

Fixes #13920