Add a note about ugni memory registration and concurrency with slurm.

(cherry picked from commit e05a5b8e99957ba2107a814e6d488deb649900c8)
Merge pull request #589 from gbtitus/doc-slurm-ugni-mem-registration-issue-1.10

Add a note about ugni memory registration and concurrency with slurm.

(cherry picked from commit e05a5b8)