Added docs on topological dependency tracking

docs/versioned_docs/v0.6/basics/reactivity.md

@@ -127,3 +127,32 @@ let root = {
 
 If we call `state.set(...)` somewhere else in our code, the text content will automatically be
 updated!
+
+## Common pitfalls
+
+Dependency tracking is *topological*, which means that reactive dependencies (like a `Signal`) must be accessed (and thus recorded as reactive dependencies) *before* the tracking scope (like a `create_effect`) returns.
+
+For example, this code won't work as intended:
+
+```rust
+create_effect(move || {
+    wasm_bindgen_futures::spawn_local(async move {
+        // This scope is not tracked because spawn_local runs on the next microtask tick (in other words, some time later).
+    };
+    // Everything that is accessed until here is tracked. Once this closure returns, nothing is tracked.
+});
+```
+
+We'll find that any `Signal`s we track in the `create_effect` won't be tracked properly in the `wasm_bindgen_futures::spawn_local`, which is often not what's intended. This problem can be gotten around by accessing reactive dependencies as needed before going into a future, or with this simple fix:
+
+```rust
+create_effect(move || {
+    let _ = signal.get(); // Where `signal` is a reactive dependency
+    wasm_bindgen_futures::spawn_local(async move {
+        // This scope is not tracked because spawn_local runs on the next microtask tick (in other words, some time later).
+    };
+    // Everything that is accessed until here is tracked. Once this closure returns, nothing is tracked.
+});
+```
+
+All we're doing there is accessing the dependency before we move into the future, which means dependency tracking should work as intended.