chore(cli): add image to gc docs and document race condition (#31925)

This might be contentious given that we don't do photos in most places but afaik we do not replicate the CLI docs on the CDK docs site so it may be safe (as it renders correctly on github).

Documents garbage collection algorithm via diagram and details potentail race condition with `REVIEW_IN_PROGRESS` stacks.

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Author: kaizencc

packages/aws-cdk/README.md

@@ -971,6 +971,26 @@ cdk gc --unstable=gc --action=delete-tagged --rollback-buffer-days=30
 
 This will delete assets that have been unused for >30 days, but will not tag additional assets.
 
+Here is a diagram that shows the algorithm of garbage collection:
+
+![Diagram of Garbage Collection algorithm](images/garbage-collection.png)
+
+#### Theoretical Race Condition with `REVIEW_IN_PROGRESS` stacks
+
+When gathering stack templates, we are currently ignoring `REVIEW_IN_PROGRESS` stacks as no template
+is available during the time the stack is in that state. However, stacks in `REVIEW_IN_PROGRESS` have already
+passed through the asset uploading step, where it either uploads new assets or ensures that the asset exists.
+Therefore it is possible the assets it references are marked as isolated and garbage collected before the stack
+template is available.
+
+Our recommendation is to not deploy stacks and run garbage collection at the same time. If that is unavoidable,
+setting `--created-buffer-days` will help as garbage collection will avoid deleting assets that are recently
+created. Finally, if you do result in a failed deployment, the mitigation is to redeploy, as the asset upload step
+will be able to reupload the missing asset.
+
+In practice, this race condition is only for a specific edge case and unlikely to happen but please open an
+issue if you think that this has happened to your stack.
+
 ### `cdk doctor`
 
 Inspect the current command-line environment and configurations, and collect information that can be useful for