documentation: xip documentation update

@@ -39,10 +39,11 @@ The block device operation is optional,
 today:
 - dcssblk: s390 dcss block device driver
 
-An address space operation named get_xip_page is used to retrieve reference
-to a struct page. To address the target page, a reference to an address_space,
-and a sector number is provided. A 3rd argument indicates whether the
-function should allocate blocks if needed.
+An address space operation named get_xip_mem is used to retrieve references
+to a page frame number and a kernel address. To obtain these values a reference
+to an address_space is provided. This function assigns values to the kmem and
+pfn parameters. The third argument indicates whether the function should allocate
+blocks if needed.
 
 This address space operation is mutually exclusive with readpage&writepage that
 do page cache read/write operations.