lib: os: spsc_pbuf: Clarify using cache management in the module

Packet buffer can be used for sharing data between cores. In that
case when any core has data cache then data cache handling must
be enabled in the module. However, it shall never be enabled when
the packet buffer is used on a single core. Adding that information
to the documentation.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>