Clarify meaning of Final in dataclasses

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-10-05.23:00:02.707>
labels = ['type-bug', 'library']
title = 'Accept Final as indicating ClassVar for dataclass'
updated_at = <Date 2022-01-21.01:21:16.689>
user = 'https://github.com/GBeauregard'

bugs.python.org fields:

activity = <Date 2022-01-21.01:21:16.689>
actor = 'med2277'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-10-05.23:00:02.707>
creator = 'GBeauregard'
dependencies = []
files = []
hgrepos = []
issue_num = 45384
keywords = []
message_count = 12.0
messages = ['403277', '403552', '403553', '403555', '403560', '403565', '403615', '403616', '403617', '403890', '403931', '411067']
nosy_count = 6.0
nosy_names = ['eric.smith', 'carljm', 'GBeauregard', 'saaketp', 'msully4321', 'med2277']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45384'
versions = []