dma-buf: Fix kerneldoc comment for struct dma_fence_array

The kerneldoc comment for struct dma_fence_array lacks a description
of the "work" member, leading to this docs-build warning:

  ./include/linux/dma-fence-array.h:54: warning: Function parameter or member 'work' not described in 'dma_fence_array'

Add a description and make the warning go away.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190116153436.3b244cda@lwn.net