vm_object.h: tweak OBJ_ONEMAPPING comment even more

The formulation that pages are allowed to have at most one mapping is
not quite clear. Nothing prevents pages from having more than one
mapping, but the flag must be cleared then.

Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential revision: https://reviews.freebsd.org/D54570