Add comments to the historacle proto

[zarazan]

Summary

There was some confusion about if the denom used is symbol or base denom:
"For historacle MedianOfHistoricMedians(ctx, denom, numStamps), is denom the token’s base denom or symbol denom? Assuming symbol for now"

Problem Definition

Robert: "we need to add more comments to the proto."

Proposal

Add comments

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[robert-zaremba]

We should also consider renaming. I was discussing similar thing for x/leverage with @toteki and we were brainstorming about few options, but didn't find ideal solution.