[PATCH] VFS: update documentation

This patch brings the now out-of-date Documentation/filesystems/vfs.txt
back to life.  Thanks to Carsten Otte, Trond Myklebust, and Anton
Altaparmakov for their help on updating this 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>