LU-930 doc: update data_version command in lfs man page

LU-930 doc: update data_version command in lfs man page

Update the lfs man page to document the [rw] options of the
data_version command, and list some of the command use cases.

Signed-off-by: Nikitas Angelinas <nangelinas@cray.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Sergey Cheremencev <c17829@cray.com>
Cray-bug-id: LUS-5890
Test-Parameters: trivial
Change-Id: I2522de9f9a987f5c085d370f6dafc0df4080f5fa
Reviewed-on: https://review.whamcloud.com/33278
Tested-by: Jenkins
Reviewed-by: Joseph Gmitter <jgmitter@whamcloud.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>