E407/demo.c (main): improve comment about interrupt priorities

It pointed to the place where we took that mysterious operation from.
Instead, make it point to a place that explains why this is needed.
fix portASSERT_IF_INTERRUPT_PRIORITY_INVALID issue

This commit contains the following changes:

- call NVIC_SetPriorityGrouping(0) to make sure the priority bits are
  set according to the liking of portASSERT_IF_INTERRUPT_PRIORITY_INVALID

- enable the assertion in xTaskGetTickCountFromISR

- copy the improved comment about NVIC_PriorityGroupConfig also to
  WM09/demo.c