Nail down and document the behavior of range expressions in RE character classes

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-07-19.19:03:20.241>
labels = ['expert-regex', 'type-bug', 'docs']
title = 'Nail down and document the behavior of range expressions in RE character classes'
updated_at = <Date 2018-07-19.19:05:07.757>
user = 'https://bugs.python.org/zwol'

bugs.python.org fields:

activity = <Date 2018-07-19.19:05:07.757>
actor = 'zwol'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Regular Expressions']
creation = <Date 2018-07-19.19:03:20.241>
creator = 'zwol'
dependencies = []
files = []
hgrepos = []
issue_num = 34156
keywords = []
message_count = 2.0
messages = ['321963', '321964']
nosy_count = 4.0
nosy_names = ['ezio.melotti', 'mrabarnett', 'docs@python', 'zwol']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34156'
versions = []