Documentation/iostats.txt: bit-size reference etc.

- correction that disk stats values are native-word-sized
  32-bit or 64-bit values, not always 32-bi values
- drop "Last modified" entry; use git for that
- fix a few typos
- change "cpu" to "CPU"

Reported-by: Linda Walsh <lkml@tlinx.org>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>