Explain PADDR and VADDR

[lukefahr]

Explain the difference between physical address and virtual address better here in the code:

https://github.com/Engr315/P7_DMA_Popcount_C/blob/c486216e611ba3c7b3a4e5b5a52f8918077ac57e/Pynq/C/user_dma.c#L170C5-L172C1