api: swagger: document BuildCache fields.
api: swagger: document BuildCache fields.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

konstruktoid/docker-upstream,cpuguy83/docker,jimjag/docker,cpuguy83/docker,konstruktoid/docker-upstream,jimjag/docker,moby/moby,moby/moby,moby/moby,Mokolea/docker,Ramzec/docker,moul/docker,Ramzec/docker,thaJeztah/docker,Mokolea/docker,mahak/docker,moul/docker,Ramzec/docker,moby/moby,mahak/docker,thaJeztah/docker,cpuguy83/docker,moul/docker,mahak/docker,konstruktoid/docker-upstream,thaJeztah/docker,Mokolea/docker,Mokolea/docker,mahak/docker,trebonian/docker,trebonian/docker,thaJeztah/docker,konstruktoid/docker-upstream,Ramzec/docker,cpuguy83/docker,jimjag/docker,jimjag/docker,trebonian/docker,trebonian/docker,moul/docker