Add fallocate_reserve option to docs

fixes bug 1095507

Grizzly's swift introduced fallocate_reserve=0 - the
number of bytes to reserve on the disk. This value can be used
to more gracefully handle out-of-disk space errors.

The patch adds the option to account/object/container conf tables

Change-Id: If5ff2ddb8f931d1bc4d7a3b28a801fa6e903d33d