Docs: Update plando_en.md with item group example (#6024)

* Update plando_en.md with item group example

Added example YAML block for item placement using an item group, including recommendation of use of `true` value with item groups to avoid unintended behaviors, with an example of the same.  Adjustments more than welcome!

* Made clarifying revision to description of Generator handling of item groups

Clarified the behavior of the Generator regarding item creation when item groups are used in plando.

worlds/generic/docs/plando_en.md

@@ -216,6 +216,28 @@ dungeon major item chests. Because the from_pool value is `false`, a copy of the
 while the originals remain in the item pool to be shuffled. The second block will place the Kokiri Sword in the Deku 
 Tree Slingshot Chest, again not from the pool.
 
+```yaml
+  plando_items:
+  # Example block - Hollow Knight
+    - items:
+        Claw : true
+      world:
+        - BobsWitness
+        - BobsRogueLegacy
+```
+This block will attempt to place all items in the Claw item group into any locations within the game slots named
+"BobsWitness" and "BobsRogueLegacy."
+
+**NOTE:** As item groups may contain items that are not currently present in the item pool, use of `true` with
+item groups, as shown here, is strongly recommended to avoid creation of unintended items.
+
+For example, the Claw item group for Hollow Knight includes Mantis_Claw, Left_Mantis_Claw, and Right_Mantis_Claw.
+Depending on a different yaml setting, the Generator will create either one Mantis_Claw item, or one each of the
+Left_Mantis_Claw and Right_Mantis_Claw items. By default, the Generator will create any missing item(s) in addition
+to using the intended item(s), resulting in placement of all three items from the item group: Mantis_Claw,
+Left_Mantis_Claw and Right_Mantis_Claw. Use of the true value, as shown in the example, restricts the Generator to
+using only the items from the item group that are already present in the item pool.
+
 ## Boss Plando
 
 This is currently only supported by A Link to the Past and Kirby's Dream Land 3. Boss plando allows a player to place a