pvf-precheck: update implementers' guide (#4612)

This commit incorporates the changes made to the runtime in the
following PRs:

- https://github.com/paritytech/polkadot/pull/4408
- https://github.com/paritytech/polkadot/pull/4457
- https://github.com/paritytech/polkadot/pull/4540
- https://github.com/paritytech/polkadot/pull/4542
- https://github.com/paritytech/polkadot/pull/4581

Note that this PR does not include the description of the PVF
pre-checker subsystem. This should be addressed within
https://github.com/paritytech/polkadot/issues/4611



Co-authored-by: sandreim <[email protected]>