MFC r350116:

Document that setmode(3) is not thread safe.

In some circumstances, setmode(3) may call umask(2) twice to retrieve
the current mode and then restore it. Between calls, the process will
have a umask of 0.

Reviewed by: markj
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D20982