The latest manual discusses displaying changelog user information in

12.1.2.5. Displaying the Changelog Index and Registered Users

mdd.*.changelog_size is also important to document, since unconsumed changelogs can cause problems.

This should also include the UNITS that this changelog size is reporting. It's a number, what does that number mean? This is a pet peeve of mine with various lustre stats and numbers from lctl. Some give an xml-ish output with units, others give a number. Chaos! If units aren't included, let's document them.