docs(core): document ContextFormed as the discriminated union it became

The rebase carried the simple-interface block from the base branch, while
this branch turned ContextFormed into a per-form discriminated union;
ContextSnapshotSection gets its own block so the page stays self-contained.

scripts/type-equiv.manifest.json

@@ -31,6 +31,11 @@
       "symbol": "ContextForm",
       "source": "packages/llm/llm/src/message.ts"
     },
+    {
+      "doc": "docs/core-data-structures/core.md",
+      "symbol": "ContextSnapshotSection",
+      "source": "packages/llm/llm/src/message.ts"
+    },
     {
       "doc": "docs/core-data-structures/core.md",
       "symbol": "ContextFormed",