Document functools.cached_property supports value updating and clearing

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-10-21.23:53:20.815>
labels = ['3.8', 'type-feature', 'library', 'docs']
title = 'Document functools.cached_property supports value updating and clearing'
updated_at = <Date 2019-10-22.03:16:10.127>
user = 'https://github.com/EpicWink'

bugs.python.org fields:

activity = <Date 2019-10-22.03:16:10.127>
actor = 'Epic_Wink'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Library (Lib)']
creation = <Date 2019-10-21.23:53:20.815>
creator = 'Epic_Wink'
dependencies = []
files = []
hgrepos = []
issue_num = 38553
keywords = []
message_count = 3.0
messages = ['355110', '355112', '355114']
nosy_count = 3.0
nosy_names = ['docs@python', 'hongweipeng', 'Epic_Wink']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38553'
versions = ['Python 3.8']