libcasper: document that most libcasper functions are not thread-safe

And neither are most libcasper services' functions, because internally
they all use cap_xfer_nvlist. cap_xfer_nvlist sends and then receives
data over a unix domain socket and associated with the cap_channel_t
argument. So absent synchronization, two threads may not use the same
cap_channel_t argument or they risk receiving the other's reply.

Sponsored by: Axcient
Reviewed by: oshogbo
Differential Revision: https://reviews.freebsd.org/D42928

(cherry picked from commit cf037972ea8863e2bab7461d77345367d2c1e054)