MFC r324683: write.2: correct maximum nbytes size for EINVAL error

In FreeBSD 11 and later debug.iosize_max_clamp defaults to 0, and the
maximum nbytes count for write(2) is SSIZE_MAX. Update the man page to
document this, and mention the sysctl that can be set to obtain the
previous behaviour.

PR: 196666
Sponsored by: The FreeBSD Foundation