Autodoc tried to scan doccomment on the module where the class defined.
But it failed to get it if the class is imported from other module.
This analyzes the target module to obtain the doccomment.
In case of the descendants of generic class, the value of
obj.__orig_bases__ is incorrect because it returns original base
arguments for the child of the generic class instead of the target
class itself.
This uses obj.__dict__ to get the correct __orig_bases__ information.
Deprecate `no_docstring` argument for `Documenter.add_content()` again.
At the first trial (#8533), it changes the behavior of
`autodoc-process-docstring` event; it is emitted unexpectedly for an
alias of class. But it brings an incompatible change to extensions.
Hence it was partially reverted at #8581.
This keeps not calling the event for an alias of class. To do that,
now `Documenter.get_doc()` can return None value.
