Expose Markdown validation selection

docs/consumers/check.md

@@ -62,6 +62,20 @@ directories; excluded directories include descendants. Symlink blobs are
 measured by link-text bytes, and gitlinks are omitted. The check applies to the
 captured candidate and reports one oversized file with its measurement as
 `file-size`; it does not measure historical or aggregate size.
+Set `Validation.Markdown` to check committed local Markdown destinations and
+supported heading fragments. Nil disables the check; an empty object enables
+it with outside-checkout destinations reported as out of scope. Set
+`EnforceCheckoutBoundary` to fail outside-checkout links. Submodule contents
+remain out of scope under either setting. The checker runs after file-size
+inspection and before `AdditionalChecks`; its nonfailure observations use the
+request's output writer. Markdown failures use the `markdown-links` identity
+and include the source location and destination.
+
+Use these selections for ReleaseKit-owned validation rather than repeating the
+standard suite, selected vet/format/distribution checks, or Markdown navigation
+checks in `AdditionalChecks` or profile `checks`. Keep release-note rules and
+consumer-specific assertions—such as tests of maintained examples—in those
+product-owned checks.
 ReleaseKit calls `AdditionalChecks` once after shared validation when supplied.
 It does not download a different toolchain automatically. If race prerequisites
 are unavailable, the `go-test` check fails rather than being skipped; a selected