docs: expand appset post selector examples (#18257)

Signed-off-by: Nicholas Morey <nicholas@morey.tech>

docs/operator-manual/applicationset/Generators-Post-Selector.md

@@ -1,8 +1,57 @@
 # Post Selector all generators
 
-The Selector allows to post-filter based on generated values using the Kubernetes common labelSelector format. In the example, the list generator generates a set of two application which then filter by the key value to only select the `env` with value `staging`:
+The `selector` field on a generator allows an `ApplciationSet` to post-filter results using [the Kubernetes common labelSelector format](https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#label-selectors) and the generated values.
 
-## Example: List generator + Post Selector
+`matchLabels` is a map of `{key,value}` pairs. This `list` generator generates a set of two `Applications`, which is then filtered using `matchLabels` to only the list element containing the key `env` with value `staging`:
+```
+spec:
+  generators:
+  - list:
+      elements:
+        - cluster: engineering-dev
+          url: https://kubernetes.default.svc
+          env: staging
+        - cluster: engineering-prod
+          url: https://kubernetes.default.svc
+          env: prod
+    selector:
+      matchLabels:
+        env: staging
+```
+
+The `list` generator + `matchLabels` selector generates a single set of parameters:
+```yaml
+- cluster: engineering-dev
+  url: https://kubernetes.default.svc
+  env: staging
+```
+
+It is also possible to use `matchExpressions` for more powerful selectors.
+
+A single `{key,value}` in the `matchLabels` map is equivalent to an element of `matchExpressions`, whose `key` field is the "key", the `operator` is "In", and the `values` array contains only the "value". So the same example using `matchExpressions` looks like:
+```yaml
+spec:
+  generators:
+  - list:
+      elements:
+        - cluster: engineering-dev
+          url: https://kubernetes.default.svc
+          env: staging
+        - cluster: engineering-prod
+          url: https://kubernetes.default.svc
+          env: prod
+      selector:
+        matchExpressions:
+          - key: env
+            operator: In
+            values:
+              - staging
+```
+
+Valid `operators` include `In`, `NotIn`, `Exists`, and `DoesNotExist`. The `values` set must be non-empty in the case of `In` and `NotIn`. 
+
+## Full Example
+In the example, the list generator generates a set of two applications, which then filter by the key value to only select the `env` with value `staging`:
 ```yaml
 apiVersion: argoproj.io/v1alpha1
 kind: ApplicationSet
@@ -36,26 +85,3 @@ spec:
         server: '{{.url}}'
         namespace: guestbook
 ```
-
-The List generator + Post Selector generates a single set of parameters:
-
-```yaml
-- cluster: engineering-dev
-  url: https://kubernetes.default.svc
-  env: staging
-```
-
-It is also possible to use `matchExpressions` for more powerful selectors.
-
-```yaml
-spec:
-  generators:
-    - clusters: {}
-      selector:
-        matchExpressions:
-          - key: server
-            operator: In
-            values:
-              - https://kubernetes.default.svc
-              - https://some-other-cluster
-```