Checking into this further, I don't think we need to add anything to the DM multipath manual.
Currently we do not document the multipathd options individually, although we do document how to find the wildcard strings and we also document raw format -- but those are general multipathd usage things rather than specific commands. Instead we refer to the man page:
"The multipathd commands can be used to administer the multipathd daemon. For information on the available multipathd commands, see the multipathd(8) man page."
This is in Section 5.12 here (The multipathd Commands):
So if we want to document this specific command we should probably document all the commands (to document this without documenting stats would be confusing), but everything is right there in the man page.
I'm closing this BZ as not a bug, since I'm the one who opened it in the first place when I editied the release note description for the feature. We mention this in the release notes.
However: I don't see this documented in the multipathd man page on the latest 7.4 build I have. I'll ping Ben about this -- but that's not part of this BZ.
If somebody things I should re-open this BZ or open a separate BZ to provide documentation for all of the multipathd commands in the DM Multipath manual (in addition to the man page), let me know, but I don't know what that would add except perhaps some examples.