[ruby/prism] Document ClassVariableWriteNode fields

(https://github.com/ruby/prism/pull/2162)

* Make ClassVariableWriteNode operator_loc non-nullable

* Document ClassVariableWriteNode fields

* Update config.yml

---------

https://github.com/ruby/prism/commit/659b133888

Co-authored-by: Kevin Newton <kddnewton@gmail.com>

prism/config.yml

@@ -1302,12 +1302,37 @@ nodes:
     fields:
       - name: name
         type: constant
+        comment: |
+          The name of the class variable, which is a `@@` followed by an [identifier](https://github.com/ruby/prism/blob/main/docs/parsing_rules.md#identifiers).
+
+              @@abc = 123     # name `@@abc`
+
+              @@_test = :test # name `@@_test`
       - name: name_loc
         type: location
+        comment: |
+          The location of the variable name.
+
+              @@foo = :bar
+              ^^^^^
       - name: value
         type: node
+        comment: |
+          The value to assign to the class variable. Can be any node that
+          represents a non-void expression.
+
+              @@foo = :bar
+                      ^^^^
+
+              @@_xyz = 123
+                       ^^^
       - name: operator_loc
-        type: location?
+        type: location
+        comment: |
+          The location of the `=` operator.
+
+              @@foo = :bar
+                    ^
     comment: |
       Represents writing to a class variable.
 

prism/prism.c

@@ -2691,7 +2691,7 @@ pm_class_variable_write_node_create(pm_parser_t *parser, pm_class_variable_read_
         },
         .name = read_node->name,
         .name_loc = PM_LOCATION_NODE_VALUE((pm_node_t *) read_node),
-        .operator_loc = PM_OPTIONAL_LOCATION_TOKEN_VALUE(operator),
+        .operator_loc = PM_LOCATION_TOKEN_VALUE(operator),
         .value = value
     };