Document that PyErr_SetInterrupt is async-signal-safe

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-18.14:31:32.964>
labels = ['type-bug', '3.8', '3.9', '3.10', 'expert-C-API', 'docs']
title = 'Document that PyErr_SetInterrupt is async-signal-safe'
updated_at = <Date 2021-02-18.14:31:32.964>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2021-02-18.14:31:32.964>
actor = 'pitrou'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'C API']
creation = <Date 2021-02-18.14:31:32.964>
creator = 'pitrou'
dependencies = []
files = []
hgrepos = []
issue_num = 43256
keywords = []
message_count = 1.0
messages = ['387234']
nosy_count = 4.0
nosy_names = ['pitrou', 'vstinner', 'docs@python', 'pablogsal']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43256'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']