LUDOC-399 dom: Fix example and clarify 64KB alignment

This patch fixes an example that incorrectly used 32KB
instead of 64KB as the dom component size.

The patch also clarifies the LFS limits for the DoM component size to
mention that the 64KB restriction is due to the minimum Lustre
stripe size.

Signed-off-by: Joseph Gmitter <joseph.gmitter@intel.com>
Change-Id: I32c2b2f46743010387196560ead7f0f880f1dcf4
Reviewed-on: https://review.whamcloud.com/31969
Tested-by: Jenkins
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>