However, I feel obliged to point out that it appears to be a
substantial majority of uses prefer the former spelling. In
fact, if you look up "acknowledgement" at http://www.m-w.com/
you get sent to the entry for "acknowledgment" (where it lists
"acknowledgement" as a variant) and a search of the online
Chicago Manual of Style at http://www.chicagomanualofstyle.org/
shows only the "acknowledgment" spelling.
We may find that most users will expect the 2e (instead of 3e)
spelling. Of course, use of a smart editor can obviate this
issue, but those entering DocBook via Notepad might be surprised.