Document lockfile id vs signature in custom node guide

Author: tobiasraabe

docs/source/how_to_guides/writing_custom_nodes.md

@@ -89,6 +89,13 @@ Here are some explanations.
   signature is a hash and a unique identifier for the node. For most nodes it will be a
   hash of the path or the name.
 
+- `signature` and lockfile `id` are different concepts.
+
+  - `signature` is the runtime identity in pytask's in-memory DAG.
+  - lockfile `id` is the portable key stored in `pytask.lock`.
+
+  For custom nodes, make sure the lockfile id stays stable and unique within a task.
+
 - The classmethod {meth}`~pytask.PickleNode.from_path` is a convenient method to
   instantiate the class.