bus_dma.9: Remove erroneous usage recommendation

It is not valid to pass BUS_SPACE_UNRESTRICTED to bus_dma_tag_create()'s
nsegments parameter as it is interpreted as a very large segment count.
Subsequent allocation operations on the tag will preallocate some multiple of
that count. BUS_SPACE_UNRESTRICTED therefore indicates something like:
malloc(infinity).

Discussed with: bcr, jhb (earlier version)