add documentation for decoding newlines in the `io` module

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-12-30.21:23:58.680>
labels = []
title = 'add documentation for decoding newlines in the `io` module'
updated_at = <Date 2021-12-30.21:25:04.160>
user = 'https://github.com/guoci'

bugs.python.org fields:

activity = <Date 2021-12-30.21:25:04.160>
actor = 'guoci'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-12-30.21:23:58.680>
creator = 'guoci'
dependencies = []
files = []
hgrepos = []
issue_num = 46209
keywords = ['patch']
message_count = 1.0
messages = ['409388']
nosy_count = 1.0
nosy_names = ['guoci']
pr_nums = ['30308']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue46209'
versions = []