LUDOC-431 lnet: doc for asymmetrical route checking

LUDOC-431 lnet: doc for asymmetrical route checking

This patch adds documentation for the asymmetrical route checking
feature, as implemented by LU-11894.
This doc is added under the Configuring LNet via lnetctl section of
the Configuring Lustre Networking chapter.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ia3214528b2bd3ae3e62dc40c06ce1b3ed565e2f3
Reviewed-on: https://review.whamcloud.com/34120
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Tested-by: Jenkins
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: Joseph Gmitter <jgmitter@whamcloud.com>