DMA-API: Clarify physical/bus address distinction

The DMA-API documentation sometimes refers to "physical addresses" when it
really means "bus addresses."  Sometimes these are identical, but they may
be different if the bridge leading to the bus performs address translation.
Update the documentation to use "bus address" when appropriate.

Also, consistently capitalize "DMA", use parens with function names, use
dev_printk() in examples, and reword a few sections for clarity.

No functional change; documentation changes only.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: James Bottomley <jbottomley@Parallels.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>

include/linux/types.h

@@ -142,6 +142,7 @@ typedef unsigned long blkcnt_t;
 #define pgoff_t unsigned long
 #endif
 
+/* A dma_addr_t can hold any valid DMA or bus address for the platform */
 #ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
 typedef u64 dma_addr_t;
 #else