Get the documentation string for an object.
All tabs are expanded to spaces. To clean up docstrings that are
indented to line up with blocks of code, any whitespace than can be
uniformly removed from the second line onwards is removed.
3.5: Documentation strings are now inherited if not overridden.
3.15: Added parameters inherit_class_doc and fallback_to_class_doc. Documentation strings on cached_property objects are now inherited if not overridden.