Add field documentation for PinnedVariableNode

Author: herwinw

config.yml

@@ -3492,8 +3492,21 @@ nodes:
     fields:
       - name: variable
         type: node
+        comment: |
+          The variable or expression used the pinned expression
+
+              foo in ^bar
+                      ^^^
+
+              foo in { timestamp: ^(Time.new(2021)..Time.new(2030)) }
+                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       - name: operator_loc
         type: location
+        comment: |
+          The location of the `^` operator
+
+              foo in ^bar
+                     ^
     comment: |
       Represents the use of the `^` operator for pinning a variable in a pattern matching expression.