man: define FI_THREAD_DOMAIN

Define another threading model - FI_THREAD_DOMAIN.

In this model, the application serializes accesses to all
objects belonging to a particular domain.

This model allows the provider to share resources among
multiple EPs. Example - a shared list receive buffers for
implementing FI_BUFFERED_RECV.

Signed-off-by: Sayantan Sur <sayantan.sur@intel.com>