Add field documentation for PinnedExpressionNode

Author: herwinw

config.yml

@@ -2465,7 +2465,7 @@ nodes:
       - name: opening_loc
         type: location?
         comment: |
-          The location of the openingbrace.
+          The location of the opening brace.
 
               foo in [*bar, baz, *qux]
                      ^
@@ -4074,12 +4074,32 @@ nodes:
       - name: expression
         type: node
         kind: non-void expression
+        comment: |
+          The expression used in the pinned expression
+
+              foo in ^(bar)
+                       ^^^
       - name: operator_loc
         type: location
+        comment: |
+          The location of the `^` operator
+
+              foo in ^(bar)
+                     ^
       - name: lparen_loc
         type: location
+        comment: |
+          The location of the opening parenthesis.
+
+              foo in ^(bar)
+                      ^
       - name: rparen_loc
         type: location
+        comment: |
+          The location of the closing parenthesis.
+
+              foo in ^(bar)
+                          ^
     comment: |
       Represents the use of the `^` operator for pinning an expression in a pattern matching expression.