gh-93417: Update mimetypes.init() doc to match the code

[bebound]

This PR is a supplement of #3062. Change the doc to match the current behavior.

After #3062, the init(files) function still loads the knownfiles when files=[]:

cpython/Lib/mimetypes.py

Lines 405 to 433 in 9b0179f

	def init(files=None):
	global suffix_map, types_map, encodings_map, common_types
	global inited, _db
	inited = True # so that MimeTypes.__init__() doesn't call us again
	if files is None or _db is None:
	db = MimeTypes()
	# Quick return if not supported
	db.read_windows_registry()
	if files is None:
	files = knownfiles
	else:
	files = knownfiles + list(files)
	else:
	db = _db
	# Lazy import to improve module import time
	import os
	for file in files:
	if os.path.isfile(file):
	db.read(file)
	encodings_map = db.encodings_map
	suffix_map = db.suffix_map
	types_map = db.types_map[True]
	common_types = db.types_map[False]
	# Make the DB a global variable now that it is fully initialized
	_db = db

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Specifying an empty list for files will prevent the system defaults from being applied: only the well-known values will be present from a built-in list. --https://docs.python.org/3/library/mimetypes.html#mimetypes.init

The above doc reflects on the old code logic:

cpython/Lib/mimetypes.py

Lines 341 to 358 in 27a8564

	def init(files=None):
	global suffix_map, types_map, encodings_map, common_types
	global inited, _db
	inited = True # so that MimeTypes.__init__() doesn't call us again
	db = MimeTypes()
	if files is None:
	if _winreg:
	db.read_windows_registry()
	files = knownfiles
	for file in files:
	if os.path.isfile(file):
	db.read(file)
	encodings_map = db.encodings_map
	suffix_map = db.suffix_map
	types_map = db.types_map[True]
	common_types = db.types_map[False]
	# Make the DB a global variable now that it is fully initialized
	_db = db

PS: I did not walk through the https://bugs.python.org/issue4963 to see whether the init(files=[]) behavior change is necessary. (Well, reverting to Python 3.7's behavior is also unlikely?) For now, we should at least update the documentation.

• Issue: Regression on initialization and new non-deterministic behavior issue in mimetypes #93417

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