The `keys` function extension is undocumented

[jg-rp]

We are missing documentation for the keys() function extension.

python-jsonpath/jsonpath/function_extensions/keys.py

Lines 7 to 8 in 8e72725

	def keys(obj: Mapping[str, object]) -> Optional[Tuple[str, ...]]:
	"""Return an object's keys, or `None` if the object has no _keys_ method."""