[Documentation] Dunder methods

[Bibo-Joshi]

Follow-up of #3907.

It should be double checked where we have dunder/magic methods of public classes that do not have a proper docstring yet. On first glance, I saw Bot.__{eq, hash}__ as well as some methods in the __error__ module.
These methods should get descriptive docstrings (even if it's just pointing to the base class) with argument descriptions where applicable.

Moreover, the :special-members: directives can be removed everywhere in the .rst files.