Add the --allow-multiple-documents flag to check-yaml hook

Yaml files with multiple `---` separators are considered "multiple-document yaml files." Without the addition of the `--allow-multiple-documents` flag to the check-yaml hook, parsing these will fail.

.pre-commit-config.yaml

@@ -7,6 +7,9 @@ repos:
     -   id: trailing-whitespace
     -   id: end-of-file-fixer
     -   id: check-yaml
+        # Multi-documents are yaml files with multiple --- separating blocks, like
+        # in our docs/parameters.yaml. We need this argument so those parse.
+        args: [--allow-multiple-documents]
     -   id: check-added-large-files
 -   repo: https://github.com/golangci/golangci-lint
     rev: v1.54.2