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>
How about: "If the call returns due to timeout, -FI_ETIMEDOUT will be returned.  The error value associated with the counter remains unchanged."?