policy: Add docs about schemas in policies

Adds docs that point to the code that validates the schemas in
policy_templates.json and that valides JSON Objects against the
schemas in policy_templates.json

Bug: 856901
Change-Id: I534c0bf58f4ff581ada4f704fc7ae0b6223f3337
Reviewed-on: https://chromium-review.googlesource.com/1116407Reviewed-by: Lutz Justen <ljusten@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570934}

components/policy/core/common/schema.h

@@ -56,6 +56,10 @@ typedef std::vector<Schema> SchemaList;
 // types of inner elements, for structured types.
 // Objects of this class refer to external, immutable data and are cheap to
 // copy.
+//
+// Schema validation is based on a subset of the JSON Schema standard.
+// TODO(crbug.com/856901): Document the supported subset of the JSON Schema
+// standard.
 class POLICY_EXPORT Schema {
  public:
   // Used internally to store shared data.

components/policy/resources/policy_templates.json

@@ -142,6 +142,11 @@
 #
 # Schemas:
 #   All policies have a key 'schema' which describes the schema of the policy.
+#   This schema supports a subset of the JSON Schema standard. For more
+#   information see //components/json_schema/json_schema_validator.h
+#   which validates the schema and //components/policy/core/common/schema.h
+#   which validates instances against the specified schema.
+#
 #   For many policies this is simply a type eg 'boolean' or 'string', but for
 #   'dict' policies this describes the types of not only the root object, but
 #   also all of its descendants. This schema data is used to validate 'dict'