m68knommu: improve short help of m68knommu/Kconfig/RAMSIZE for '0' case

While it is explained in the long help text, meaning of '0' for RAMSIZE
is easily overlooked because is not mentioned in the short help text.
Add that.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>