"posix: Maps POSIX FS semantics to NT semantics" is listed two times in the documentation of the "ntvfs handler" configuration parameter.
I'm not familiar with this paramter, but there seems to be a "cifsposix", too, which isn't documented yet. So I assume one of these two entries should be changed to / replaced by a proper description of "cifsposix".
Btw: there is a small typo in source4/ntvfs/cifs_posix_cli/README:
nfvfs handler = cifsposix
Seems to have been fixed as of now in master. Either way this parameter is only for the developer-only NTVFS file server.