Add *BSD pthread API notes

[etcwilde]

Description

FreeBSD and OpenBSD do not annotate the pthread APIs with nullability information. We should use API notes to specify that in a platform overlay for these platforms.

Additional information

Workaround PR posted in #81366

[jakepetroules]

This would be a source breaking change. Are there any concerns with that? Or we'd just expect everyone to adopt, alongside using #if compiler if their codebase needed to be compatible with multiple versions?