I am dumb and can't spell (#1366)

* rename implementor's guide to implementer's guide

* fix typos in more places

polkadot/roadmap/implementers-guide/src/node/utility/candidate-validation.md

@@ -0,0 +1,23 @@
+# Candidate Validation
+
+This subsystem is responsible for handling candidate validation requests. It is a simple request/response server.
+
+A variety of subsystems want to know if a parachain block candidate is valid. None of them care about the detailed mechanics of how a candidate gets validated, just the results. This subsystem handles those details.
+
+## Protocol
+
+Input: [`CandidateValidationMessage`](../../types/overseer-protocol.md#validation-request-type)
+
+Output: Validation result via the provided response side-channel.
+
+## Functionality
+
+Given the hashes of a relay parent and a parachain candidate block, and either its PoV or the information with which to retrieve the PoV from the network, spawn a short-lived async job to determine whether the candidate is valid.
+
+Each job follows this process:
+
+- Get the full candidate from the current relay chain state
+- Check the candidate's proof
+   > TODO: that's extremely hand-wavey. What does that actually entail?
+- Generate either `Statement::Valid` or `Statement::Invalid`. Note that this never generates `Statement::Seconded`; Candidate Backing is the only subsystem which upgrades valid to seconded.
+- Return the statement on the provided channel.