Add async backing parameters and fix image references

docs/maintain/maintain-guides-async-backing.md

@@ -13,7 +13,7 @@ slug: ../maintain-guides-async-backing
 This guide is relevant for cumulus based parachain projects started in 2023 or before. Later
 projects should already be async backing compatible. If starting a new parachain project, please use
 an async backing compatible template such as
-[`cumulus/parachain-template`](https://github.com/paritytech/cumulus/tree/rh-test-async-backing).
+[`cumulus/parachain-template`](https://github.com/paritytech/polkadot-sdk/tree/master/cumulus/parachain-template).
 
 :::
 
@@ -195,7 +195,7 @@ This phase consists of changes to your parachain’s runtime that activate async
 
 1. Increase the maximum `UNINCLUDED_SEGMENT_CAPACITY` in `runtime/src/lib.rs`.
 
-![Unincluded-segment-capacity](../assets/async/async-backing-unincluded-segment.png)
+![Unincluded-segment-capacity](../assets/async/async-backing-unincluded-segment-capacity.png)
 
 3. Decrease `MILLISECS_PER_BLOCK` to 6000.
 
@@ -214,6 +214,20 @@ This phase consists of changes to your parachain’s runtime that activate async
 
 ![minimum-period](../assets/async/async-backing-minimum-period.png)
 
+6. Check parameters: double-check that the relay-chain configuration contains the following three
+   parameters (especially when testing locally e.g. with zombienet):
+
+```json
+"async_backing_params": {
+    "max_candidate_depth": 3,
+    "allowed_ancestry_len": 2
+},
+"scheduling_lookahead": 2
+```
+
+⚠️ `scheduling_lookahead` must be set to 2, otherwise parachain block times will degrade to worse
+than with sync backing! ⚠️
+
 ## Timing by Block Number
 
 With asynchronous backing it will be possible for parachains to opt for a block time of 6 seconds