pthread_sigmask needs SIG_BLOCK behaviour explaination

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-03-28.09:48:02.840>
labels = ['interpreter-core', 'type-bug', '3.10', '3.11']
title = 'pthread_sigmask needs SIG_BLOCK behaviour explaination'
updated_at = <Date 2022-04-05.19:28:09.388>
user = 'https://github.com/rpurdie'

bugs.python.org fields:

activity = <Date 2022-04-05.19:28:09.388>
actor = 'rpurdie'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2022-03-28.09:48:02.840>
creator = 'rpurdie'
dependencies = []
files = []
hgrepos = []
issue_num = 47139
keywords = []
message_count = 4.0
messages = ['416154', '416757', '416794', '416808']
nosy_count = 2.0
nosy_names = ['gregory.p.smith', 'rpurdie']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue47139'
versions = ['Python 3.10', 'Python 3.11']