powerpc: Update NUMA Kconfig description & help text

Update the NUMA Kconfig description to match other architectures, and
add some help text. Shamelessly borrowed from x86/arm64.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20201124120547.1940635-3-mpe@ellerman.id.au