LU-11638 admin: debug space management/missing objects

LU-11638 admin: debug space management/missing objects

Improve troubleshooting section - formatting, improve incorrect
content, update to newer descriptions of how the MDS and OSS
precreate objects, and what they now do if there is a problem.

Improve the description of free space management.  Add example usage
of "open_files" and "lsof" and "lfs df" and "lfs df -i" output.

Improve Jira search description, and update the URL for the
lustre-discuss mailing list.

Fix some cross-references to be more descriptive (lots more could
be done in this area).

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I19dadb826d2fcf5e298f04a4c660c7a8857995c9
Reviewed-on: https://review.whamcloud.com/33629
Reviewed-by: Joseph Gmitter <jgmitter@whamcloud.com>
Tested-by: Jenkins