man: fi_cntr_wait, fi_cq_sread clarify timeout

When these calls return due to timeout, they should return a
specific value, vs. a generic error. This indicates to the app
that it doesn't have to read the error queue.

Signed-off-by: Sayantan Sur <sayantan.sur@intel.com>
Merge pull request #237 from sayantansur/fi-cntr-wait-timeout

man: fi_cntr_wait, fi_cq_sread clarify timeout