f2fs: delete duplicate information on sysfs nodes

This patch merges the sysfs node documentation present in
Documentation/filesystems/f2fs.txt and
Documentation/ABI/testing/sysfs-fs-f2fs
and deletes the duplicate information from
Documentation/filesystems/f2fs.txt. This is to prevent having to update
both files when a new sysfs node is added for f2fs.
The patch also makes minor formatting changes to
Documentation/ABI/testing/sysfs-fs-f2fs.

Signed-off-by: Hridya Valsaraju <hridya@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>