Add documentation for the BPF spinlock-related helpers to the doc in
bpf.h. I added the constraints and restrictions coming with the use of
spinlocks for BPF: not all of it is directly related to the use of the
helper, but I thought it would be nice for users to find them in the man
page.

This list of restrictions is nearly a verbatim copy of the list in
Alexei's commit log for those helpers.
Signed-off-by: NQuentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>