ipm.h: fix erroneous edit of documentation

Patch 3010de98 changed the
documentation for the ipm_send() 'wait' parameter to something
that is not what the parameter does, and removed some helpful
ancillary information.

Change-Id: Id79950207e9b3d8cfe8f664cbf45fbfa24069dea
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>