[doc] Add re.sub examples with `lamda`

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-12-08.13:48:29.326>
labels = ['type-feature', '3.9', 'docs']
title = '[doc] add more examples and additional explanation to re.sub'
updated_at = <Date 2020-12-08.13:48:29.326>
user = 'https://bugs.python.org/harvastum'

bugs.python.org fields:

activity = <Date 2020-12-08.13:48:29.326>
actor = 'harvastum'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2020-12-08.13:48:29.326>
creator = 'harvastum'
dependencies = []
files = []
hgrepos = []
issue_num = 42601
keywords = []
message_count = 1.0
messages = ['382736']
nosy_count = 2.0
nosy_names = ['docs@python', 'harvastum']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue42601'
versions = ['Python 3.9']