[3.10] bpo-27718: Fix help for the signal module (GH-30063) (GH-30080)

[3.10] bpo-27718: Fix help for the signal module (GH-30063) (GH-30080)

Functions signal(), getsignal(), pthread_sigmask(), sigpending(),
sigwait() and valid_signals() were omitted.

If __all__ is not defined all non-builtin functions should have
correct __module__.
(cherry picked from commit e08c0d8eec528f1d7a282ee19bcadb9aae9ec123)
(cherry picked from commit e55deaabd8de338138cf29aea6890996e794c997)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>