Fix documentation for __instancecheck__

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-10-27.20:14:41.592>
labels = ['type-feature', '3.8', 'docs']
title = 'Fix documentation for __instancecheck__'
updated_at = <Date 2020-12-15.13:52:24.473>
user = 'https://github.com/Rhodolite'

bugs.python.org fields:

activity = <Date 2020-12-15.13:52:24.473>
actor = 'eirrgang'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2018-10-27.20:14:41.592>
creator = 'joydiamond'
dependencies = []
files = []
hgrepos = []
issue_num = 35083
keywords = []
message_count = 5.0
messages = ['328658', '328687', '330286', '383046', '383057']
nosy_count = 6.0
nosy_names = ['docs@python', 'mdk', 'xtreak', 'joydiamond', 'donovick', 'eirrgang']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35083'
versions = ['Python 2.7', 'Python 3.8']