Add some documentation to xnn_allocation_type

PiperOrigin-RevId: 456613267
diff --git a/src/xnnpack/subgraph.h b/src/xnnpack/subgraph.h
index d3fcb87..80c3a4b 100644
--- a/src/xnnpack/subgraph.h
+++ b/src/xnnpack/subgraph.h
@@ -108,8 +108,11 @@
 
 enum xnn_allocation_type {
   xnn_allocation_type_invalid = 0,
+  /// Static data that is provided by caller, needs to outlive the xnn_runtime.
   xnn_allocation_type_static,
+  /// Lives in XNNPACK-managed internal workspace.
   xnn_allocation_type_workspace,
+  /// Non-static data that is external to the runtime, provided by caller, specified in xnn_setup_runtime.
   xnn_allocation_type_external,
 };