Finalize the documentation of block diffusion pipelining via delayed validation

Block diffusion pipelining via delayed validation has been implemented in input-output-hk/ouroboros-network#3688, but the corresponding discussion paper (see input-output-hk/ouroboros-network#3579) has not been finalized. The goal of this issue is to modify it to accurately describe the already-shipped implementation, comparing it with other internal iterations of the document.

From a cursory glance, it looks like it can mostly stay as it is, excluding the implementation section, which still describes a tentative approach we did not end up using.

Also, it should explain https://github.com/IntersectMBO/ouroboros-consensus/pull/955