Inherit docstrings from parent classes

[has2k1]

This change makes it so that subclass methods that do not have
docstrings inherit a docstring from the first parent class whose
overriden method has a docstring.

It also improves the case for the options

include_empty: false
include_inherited: true

where inherited subclass methods are omitted if they have no docstring,
yet a parent class method has a docstring.

e.g.

class Base:
    def m1(): ...
        """Calculate m1"""

    def m2(): ...
        """Calculate m2"""

class Derived(Base):
    def m2(): ...

If the intention is to document Derived and include the inherited
methods, it would be a surprise if Derived.m2 is omitted or it is
included but it has no docstring. As a result to have an option like
include_inherited_docstring that can be false does not align
with the users expected intentions.

[has2k1]

The tests that fail were broken by griffe v0.42.0.