Follow-up from "Dynamic switch between brassaia and irmin"

The following discussion from !13054 (merged) should be addressed:

• @mattiasdrp started a discussion: (+3 comments)

  I guess there's no way to make this visible outside of a comment in product_octez? I try adding a ~description stanza to the declaration but it doesn't seem to change anything.

Summary: We need to improve the manifest to make sure that context_ops is used instead of concrete instantiations (like context_brassaia or irmin)