DOC: update xip method info

xip documentation updated:
- change "get_xip_page" to "get_xip_mem";
- explain changed function parameters

Signed-off-by: Marco Stornelli <marco.stornelli@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>