[PATCH] VFS: update overview document

This patch updates the Documentation/filesystems/vfs.txt document.  I
rearranged and rewrote parts of the introduction chapter and added better
headings for each section.  I also added a description for the inode
rename() operation which was missing and added links to some useful
external VFS documentation.
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>