`--embed-mixins`
(https://github.com/ruby/rdoc/pull/842

)

* Embed mixed-in methods and constants with `--embed-mixins`

When `--embed-mixins` option is set:

- methods from an `extend`ed module are documented as singleton methods
- attrs from an `extend`ed module are documented as class attributes
- methods from an `include`ed module are documented as instance methods
- attrs from an `include`ed module are documented as instance attributes
- constants from an `include`ed module are documented

Sections are created when needed, and Darkfish's template annotates
each of these mixed-in CodeObjects. We also respect the mixin methods'
visibility.

This feature is inspired by Yard's option of the same name.

* Add comment to document why we set object visibility

Co-authored-by: Stan Lo <stan001212@gmail.com>

* Add the mixin_from attribute to CodeObject's initializer

* Add test coverage for private mixed-in attributes.

---------

https://github.com/ruby/rdoc/commit/481c2ce660



Co-authored-by: Stan Lo <stan001212@gmail.com>