Add structure documentation for metadata
Add structure documentation for metadata

Adds comment showing the structure of the buckets
as stored in boltdb. Includes all bucket names, key names
and descriptions of the stored values.

Moves the metadata comment to package level to be exposed
in the godoc overview.

Signed-off-by: Derek McGowan <e1c79a582b6629e6b39e9679f4bb964d25db4aa8@mcgstyle.net>

mikebrow/containerd,vdemeester/containerd,dmcgowan/containerd,crosbymichael/containerd,estesp/containerd,thaJeztah/containerd,vdemeester/containerd,thaJeztah/containerd,containerd/containerd,mikebrow/containerd,dmcgowan/containerd,crosbymichael/containerd,containerd/containerd,estesp/containerd