Fixes #4847: Move r32465 to v1.8 branch (usnic: update connectivity checker help)

---svn-pre-commit-ignore-below---

(shifted r32465 patch to OMPI tree to apply to v1.8 branch)

r32465 [[BR]]
usnic: update connectivity checker help message

Show an example of using the btl_usnic_connectivity_map option.  Also,
mention that another reason for the "total connectivity failure" may
be due to asymmetric / unexpected routing.

Reviewed by Dave Goodell.

Fixes Cisco bug CSCuq29583.

cmr=v1.8.2:reviewer=ompi-rm1.8