[doc] clarify what data the mimetypes.MimeTypes class uses

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-10-31.19:05:11.178>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = '[doc] clarify what data the mimetypes.MimeTypes class uses'
updated_at = <Date 2022-01-14.14:18:37.895>
user = 'https://bugs.python.org/toonn'

bugs.python.org fields:

activity = <Date 2022-01-14.14:18:37.895>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-10-31.19:05:11.178>
creator = 'toonn'
dependencies = []
files = []
hgrepos = []
issue_num = 38656
keywords = []
message_count = 13.0
messages = ['355763', '355781', '355782', '355841', '356106', '356853', '356861', '356865', '356866', '356867', '356868', '373795', '373832']
nosy_count = 9.0
nosy_names = ['r.david.murray', 'steve.dower', 'The Compiler', 'ammar2', 'michael-lazar', 'dhess', 'xtreak', 'toonn', 'andrei.avk']
pr_nums = ['27750']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38656'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']