doc: "module" of a warning might be its filename

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-07-19.23:02:26.024>
labels = ['type-bug', 'library', '3.9']
title = 'doc: "module" of a warning might be its filename'
updated_at = <Date 2019-07-19.23:02:26.024>
user = 'https://github.com/blueyed'

bugs.python.org fields:

activity = <Date 2019-07-19.23:02:26.024>
actor = 'blueyed'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-07-19.23:02:26.024>
creator = 'blueyed'
dependencies = []
files = []
hgrepos = []
issue_num = 37634
keywords = []
message_count = 1.0
messages = ['348201']
nosy_count = 1.0
nosy_names = ['blueyed']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37634'
versions = ['Python 3.9']