Update Paras module to account for candidate-descriptor / commitments split

[rphmeier]

After #1276 - The guide section on the Paras module has diverged from the implementation ever since #1294 , where a storage optimization was made. The storage should be made to match what is described in the guide.

[rphmeier]

Specifically, this is about the CandidatePendingAvailability logic. Although it makes more sense to have the CandidatePendingAvailability store a CandidateDescriptor instead of CandidateReceipt, as is written in the guide. So a small update to the guide to do that would be good as well alongside changing the code to match.