man: remove references to fi_sync

Remove references to fi_sync in the manpages since it
has been removed from the API.

Applications can use counters directly to wait until issued
ops are completed.

The comment about lazy deregistration on fi_close(mr) was
also removed. Asynchronous registration is now handled using
domain events.

(Currently, it is not clear what event is caused during lazy
deregistration).

Signed-off-by: Sayantan Sur <sayantan.sur@intel.com>