Add missing bnonce prose

We have a prose description of `bleader` in the Shelley spec, but nowhere is `bnonce` ever described.

Notably, `bleader` is described as "indicates a possible leader for this slot", but it's not clear how a similar definition would apply to `bnonce` and if `bnonce` is the same thing as the `nonce_vrf` from the binary spec of previous eras

Start with the Shelley spec prose for `bleader`, then inspect where `bnonce` appears and compare it with `nonce_vrf` in the binary spec for previous eras. Determine the intended relationship and add a clear prose description of `bnonce` to the relevant specification section.

Written by the indexing model from the issue text.