m68k: mac: Update Kconfig help

m68k: mac: Update Kconfig help

There is still some missing hardware support that affects all models,
such as sound chip and localtalk support. However, many models are well
supported, including the Quadra 800 emulated by QEMU. Missing hardware
support is mostly documented at the web site, so add the URL.

Cc: Joshua Thompson <funaho@jurai.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Link: https://lore.kernel.org/r/bb327f05f8fb61eeb332cc2ba4e8335570976474.1605847196.git.fthain@telegraphics.com.au
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>