LUDOC-367 lnet: Initial MR Documentation

LUDOC-367 lnet: Initial MR Documentation

Initial documention for the LNet multi-rail feature added
in LU-7734.

lnetctl commands have been updated in section 9 where
appropriate to reflect MR changes and syntax.

A new section has also been created for current and
future multi-rail specific examples and content.

Signed-off-by: Joseph Gmitter <joseph.gmitter@intel.com>
Change-Id: Ie1d5a174f3fd02e39e011d349828654dfa8d540c
Reviewed-on: https://review.whamcloud.com/27687
Tested-by: Jenkins
Reviewed-by: Sonia Sharma <sonia.sharma@intel.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>