Document atomicity for read(2) and write(2).

Take part of the text from POSIX 2018 edition and describe the
atomicity requirements for read and write syscalls. See p1003.1-2018,
Vol.2, 2.9.7 Threads interaction with Regular File Operations.

Reviewed by: asomers
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential revision: https://reviews.freebsd.org/D20867