Update parse-tree-patterns.md (#665)

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docs/contributing/parse-tree-patterns.md

@@ -6,15 +6,16 @@ We have a small domain-specific language that we use to define patterns to look
 
 `GRAND_PARENT_TYPE.*.CHILD_TYPE[FIELD]!?`
 
-| Syntax              | Description                                                                                                                       |
-| ------------------- | --------------------------------------------------------------------------------------------------------------------------------- |
-| TYPE                | Match node type<br/>`node.type`                                                                                                   |
-| Dot operator(`.`)   | Type hierarchy between parent and child                                                                                           |
-| Wildcard op(`*`)    | Match any type                                                                                                                    |
-| Negation op(`~`)    | Match any type other than what is specified after `~`                                                                             |
-| Important op(`!`)   | Use this node as result instead of parent.<br/>By default the leftmost/top node is used                                           |
-| Optional op(`?`)    | Node is optional. Will match if available.                                                                                        |
-| Field op(`[field]`) | Get child field node for resulting node.<br/>Only evaluated on the resulting node at the end.<br/>`node.childForFieldName(field)` |
+| Syntax              | Description                                                                             |
+| ------------------- | --------------------------------------------------------------------------------------- |
+| TYPE                | Match node type<br/>`node.type`                                                         |
+| Dot operator(`.`)   | Type hierarchy between parent and child                                                 |
+| Wildcard op(`*`)    | Match any type                                                                          |
+| Negation op(`~`)    | Match any type other than what is specified after `~`                                   |
+| Important op(`!`)   | Use this node as result instead of parent.<br/>By default the leftmost/top node is used |
+| Optional op(`?`)    | Node is optional. Will match if available.                                              |
+| Field op(`[field]`) | Return child node at given field.<br/>`node.childForFieldName(field)`                   |
+| Index op(`[n]`)     | Return nth named child node.                                                            |
 
 ## Multiple patterns