Use enum value init expression instead of numeric value in doc format

src/entity.cpp

@@ -205,6 +205,7 @@ struct Entity {
 			i32 field_group_index;
 			CommentGroup *docs;
 			CommentGroup *comment;
+			Ast *init_expr; // only used for enum values
 		} Constant;
 		struct {
 			Ast *type_expr; // only used for some variables within procedure bodies