Documentation/filesystems/vfs.txt: update file_operations documentation

->readv, ->writev and ->sendfile have been removed while ->show_fdinfo
has been added. The documentation should reflect this.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>