Document namespaceId cross-wiki filtering caveat in the graph model

Follows-up to #946

Clarifies that the new `namespaceId` page property uses MediaWiki's canonical
namespace IDs: built-in namespaces have the same ID on every wiki, so filtering
behaves consistently when the graph is shared by multiple wikis, while custom
namespaces defined via $wgExtraNamespaces get per-wiki IDs and should be paired
with wiki_id for unambiguous cross-wiki filtering.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 2 people

docs/reference/graph-model.md

@@ -47,6 +47,11 @@ page ids are only unique within a single wiki. Each page node therefore carries
 `creationTime` and `lastUpdated` are stored as Neo4j datetime values (ISO 8601), converted from MediaWiki's
 `YmdHis` timestamp format.
 
+`namespaceId` holds MediaWiki's canonical namespace ID. Built-in namespaces (e.g. `0` main, `12` Help, `14`
+Category) have the same ID on every wiki, so filtering by `namespaceId` behaves consistently across a graph
+shared by multiple wikis. Custom namespaces defined via `$wgExtraNamespaces` get per-wiki IDs that may differ in
+meaning between wikis, so pair `namespaceId` with `wiki_id` to filter those unambiguously.
+
 ## Subject Nodes
 
 Each Subject stored on a page gets a `:Subject` node. Subject nodes carry two labels: `Subject` and the name of