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 <54316454+sandreim@users.noreply.github.com>

polkadot/roadmap/implementers-guide/src/runtime-api/pvf-prechecking.md

@@ -0,0 +1,22 @@
+# PVF Pre-checking
+
+> ⚠️ This runtime API was added in v2.
+
+There are two main runtime APIs to work with PVF pre-checking.
+
+The first runtime API is designed to fetch all PVFs that require pre-checking voting. The PVFs are
+identified by their code hashes. As soon as the PVF gains required support, the runtime API will
+not return the PVF anymore.
+
+```rust
+fn pvfs_require_precheck() -> Vec<ValidationCodeHash>;
+```
+
+The second runtime API is needed to submit the judgement for a PVF, whether it is approved or not.
+The voting process uses unsigned transactions. The [`PvfCheckStatement`](../types/pvf-prechecking.md) is circulated through the network via gossip similar to a normal transaction. At some point the validator
+will include the statement in the block, where it will be processed by the runtime. If that was the
+last vote before gaining the super-majority, this PVF will not be returned by `pvfs_require_precheck` anymore.
+
+```rust
+fn submit_pvf_check_statement(stmt: PvfCheckStatement, signature: ValidatorSignature);
+```