doc: fix an incorrect suggestion to pass NULL for PCI like buses

	cpu_addr = pci_alloc_consistent(pdev, size, &dma_handle);

where pdev is a struct pci_dev *. You should pass NULL for PCI like buses

where devices don't have struct pci_dev (like ISA, EISA).  This may be

called in interrupt context. 

where pdev is a struct pci_dev *. This may be called in interrupt context.

You should use dma_alloc_coherent (see DMA-API.txt) for buses

where devices don't have struct pci_dev (like ISA, EISA).

This argument is needed because the DMA translations may be bus

specific (and often is private to the bus which the device is attached