Add autoproperty to the documentation

[SwampFalc]

The current documentation for ext.autodoc does not mention the existence of autoproperty.

Since I spent a few hours trying to find a workaround, only for a random comment in a thread to reveal that it actually exists, I would like this documentation error to be fixed.

[tk0miya]

Indeed, it must be added.

[astrojuanlu]

For reference, the .. autoxxx directives are registered here:

sphinx/sphinx/application.py

Line 1106 in 186bbc1

self.add_directive('auto' + cls.objtype, AutodocDirective, override=override)

and the PropertyDocumenter class is defined here:

sphinx/sphinx/ext/autodoc/__init__.py

Lines 2555 to 2559 in 1f860cb

	class PropertyDocumenter(DocstringStripSignatureMixin, ClassLevelDocumenter): # type: ignore
	"""
	Specialized Documenter subclass for properties.
	"""
	objtype = 'property'

and was added in Sphinx 2.1.0.