Document the optional callback parameter of weakref.WeakMethod

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-04-04.21:03:28.457>
labels = ['type-feature', 'library', '3.9', '3.10', '3.11']
title = 'Document the optional callback parameter of weakref.WeakMethod'
updated_at = <Date 2022-04-05.05:34:31.580>
user = 'https://github.com/maggyero'

bugs.python.org fields:

activity = <Date 2022-04-05.05:34:31.580>
actor = 'rhettinger'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-04-04.21:03:28.457>
creator = 'maggyero'
dependencies = []
files = []
hgrepos = []
issue_num = 47220
keywords = []
message_count = 2.0
messages = ['416697', '416753']
nosy_count = 3.0
nosy_names = ['rhettinger', 'pitrou', 'maggyero']
pr_nums = ['25491']
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue47220'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']