If possible, I'd prefer to retain some kind of copyleft, i.e, it should not be allowed to remix the manual and then require a per-copy licensing fee for the result, or apply drm-madness, and things like that.
Hm. Do you really feel that the API documentation has such intrisic value that this would be a realistic "threat"? I fail to see a scenario where such a clause would benefit either you or humanity at large...