document whether io.TextIOBase.readline(size>0) will always read the full newline

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-27.02:15:37.431>
labels = ['docs']
title = 'document whether io.TextIOBase.readline(size>0) will always read the full newline'
updated_at = <Date 2021-07-03.03:11:32.241>
user = 'https://bugs.python.org/calestyo'

bugs.python.org fields:

activity = <Date 2021-07-03.03:11:32.241>
actor = 'andrei.avk'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2021-02-27.02:15:37.431>
creator = 'calestyo'
dependencies = []
files = []
hgrepos = []
issue_num = 43336
keywords = []
message_count = 4.0
messages = ['387758', '387759', '388935', '396887']
nosy_count = 4.0
nosy_names = ['benjamin.peterson', 'docs@python', 'calestyo', 'andrei.avk']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43336'
versions = []