cubicweb-queueing #2894664 Document the public API [validation pending]

It must be made clear which API is to be used by a final cube that needs to use queueing.

prioritynormal
typeenhancement
done in0.4.0
load1.000
load left0.000
closed by#54dfe1c57ab8 [doc] Add basic cube usage documentation and test code (closes #2894664).
patch[doc] Add basic cube usage documentation and test code (closes #2894664). [applied]