doc:adding "--allow-shrink" in decreasing the size of the rbd block to distinguish...

doc:adding "--allow-shrink" in decreasing the size of the rbd block to distinguish from the increasing option

In the original file, the increasing and decreaing of the size of the rbd block shares the same option:
    "rbd resize --size 2048 foo".
However, it is not proper, as the "--allow-shrink" needs to be added while decreasing the size of the rbd block.
As a result, it is necessary to make a distinguish between these two options as follows:
    "rbd resize --size 2048 foo (to increase)"
    "rbd resize --size 2048 foo --allow-shrink (to decrease)"

Signed-off-by: Yehua <chen.yehua@h3c.com>