docs: clarify valid URNs

proto/substrait/extensions/extensions.proto

@@ -29,6 +29,7 @@ message SimpleExtensionURN {
 
   // The extension URN that uniquely identifies this extension. This must follow the
   // format extension:<OWNER>:<ID> and serves as the "namespace" of this extension.
+  // The URN must be valid RFC 8141 format without the urn: prefix.
   string urn = 2;
 }
 

site/docs/extensions/index.md

@@ -13,13 +13,15 @@ Some kinds of primitives are so frequently extended that Substrait defines a sta
 * Window Functions
 * Table Functions
 
-To extend these items, developers can create one or more YAML files that describe the properties of each of these extensions. Each YAML file must include a required `urn` field that uniquely identifies the extension. While these identifiers are URN-like but not technically URNs (they lack the `urn:` prefix), they will be referred to as `extension URNs` for clarity.
+To extend these items, developers can create one or more YAML files that describe the properties of each of these extensions. Each YAML file must include a required `urn` field that uniquely identifies the extension. While these identifiers are URN-like but not technically URNs (they lack the `urn:` prefix), they will be referred to as `extension URNs` for clarity. These URNs must be valid [RFC 8141](https://www.rfc-editor.org/rfc/rfc8141.html) format without the `urn:` prefix.
 
 This extension URN uses the format `extension:<OWNER>:<ID>`, where:
 
 - `OWNER` represents the organization or entity providing the extension and should follow [reverse domain name convention](https://en.wikipedia.org/wiki/Reverse_domain_name_notation) (e.g., `io.substrait`, `com.example`, `org.apache.arrow`) to prevent name collisions
 - `ID` is the specific identifier for the extension (e.g., `functions_arithmetic`, `custom_types`)
 
+Thus, if `extension:<OWNER>:<ID>` is our URN, then `urn:extension:<OWNER>:<ID>` must be a valid [RFC 8141 URN](https://www.rfc-editor.org/rfc/rfc8141.html).
+
 The YAML file is constructed according to the [YAML Schema](https://github.com/substrait-io/substrait/blob/main/text/simple_extensions_schema.yaml). Each definition in the file corresponds to the YAML-based serialization of the relevant data structure. If a user only wants to extend one of these types of objects (e.g. types), a developer does not have to provide definitions for the other extension points.
 
 A Substrait plan can reference one or more YAML files via their extension URN. In the places where these entities are referenced, they will be referenced using an extension URN + name reference. The name scheme per type works as follows:

site/docs/serialization/binary_serialization.md

@@ -22,7 +22,7 @@ For simple extensions, a plan references the extension URNs associated with the
 
 Simple extensions within a plan are split into three components: an extension URN, an extension declaration and a number of references.
 
-* **Extension URN**: A unique identifier for the extension following the format `extension:<OWNER>:<ID>` that identifies a YAML document specifying one or more specific extensions. Declares an anchor that can be used in extension declarations.
+* **Extension URN**: A unique identifier for the extension following the format `extension:<OWNER>:<ID>` that identifies a YAML document specifying one or more specific extensions. Declares an anchor that can be used in extension declarations. The URN with the `urn:` prefix added must conform to [RFC 8141](https://www.rfc-editor.org/rfc/rfc8141.html).
 * **Extension Declaration**: A specific extension within a single YAML document. The declaration combines a reference to the associated extension URN along with a unique key identifying the specific item within that YAML document (see [Function Signature](../extensions/index.md#function-signature)). It also defines a declaration anchor. The anchor is a plan-specific unique value that the producer creates as a key to be referenced elsewhere.
 * **Extension Reference**: A specific instance or use of an extension declaration within the plan body.