arch: riscv: switch: Add a comment on the return of z_riscv_switch

When returning from z_riscv_switch, depending on whether the thread that
has just been swapped in was earlier swapped out synchronously (i.e. via
regular function call) or asynchronously (i.e. via exception/irq) we
will return to arch_switch() or __irq_wrapper respectively. Comment this
fact for clarity.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>