NTB: Fix documentation for ntb_link_is_up

There was a copy and paste error in the documentation for
ntb_link_is_up.  The long description was mistakenly copied from
ntb_link_set_trans.

This adds the appropriate long description for ntb_link_is_up.

Reported-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Allen Hubbe <Allen.Hubbe@emc.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>