update documentation

Co-authored-by: Thomas Grützmacher <[email protected]>

include/ginkgo/core/config/config.hpp

@@ -47,24 +47,25 @@ class pnode;
  *    Cg solver. Note. the template type is given by the another entry or from
  *    the type_descriptor.
  * 5. the data type uses fixed-width representation
- *    void, int32, int64, float32, float64, complex<float32>, complex<float64>
- * 6. We use [real, imag] to represent complex value. If it only contains one
- *    value or without [], we will treat it as the complex number with imaginary
+ *    int32, int64, float32, float64, complex<float32>, complex<float64>.
+ *    note: we have also allow `void` additionally in type_descriptor to specify
+ *          file must contain the value/index type config.
+ * 6. We use [real, imag] to represent complex values. If it only contains one
+ *    value or none [], we will treat it as a complex number with an imaginary
  *    part = 0.
- * 7. In many cases, the parameter allows vector input and we handle it by array
- *    of property tree. If the array only contains one object, users can
- *    directly provide the object without putting it into an array.
+ * 7. In many cases, the parameter allows array input. If the array only
+ *    contains one object, users can directly provide the object without putting
+ *    it into an array.
  *    `"criteria": [{...}]` and `"criteria": {...}` are the same.
  *
- * The configuration needs
- * to specify the resulting type by the field:
+ * The configuration needs to specify the resulting type by the field:
  * ```
  * "type": "some_supported_ginkgo_type"
  * ```
  * The result will be a deferred_factory_parameter,
  * which can be thought of as an intermediate step before a LinOpFactory.
- * Providing the result an Executor through the function `.on(exec)` will then
- * create the factory with the parameters as defined in the configuration.
+ * Providing an Executor through the function `.on(exec)` will then create the
+ * factory with the parameters as defined in the configuration.
  *
  * Given a configuration that is defined as
  * ```

include/ginkgo/core/solver/cg.hpp

@@ -16,6 +16,7 @@
 #include <ginkgo/core/base/types.hpp>
 #include <ginkgo/core/config/config.hpp>
 #include <ginkgo/core/config/registry.hpp>
+#include <ginkgo/core/config/type_descriptor.hpp>
 #include <ginkgo/core/log/logger.hpp>
 #include <ginkgo/core/matrix/dense.hpp>
 #include <ginkgo/core/matrix/identity.hpp>
@@ -75,6 +76,18 @@ class Cg : public EnableLinOp<Cg<ValueType>>,
     GKO_ENABLE_LIN_OP_FACTORY(Cg, parameters, Factory);
     GKO_ENABLE_BUILD_METHOD(Factory);
 
+    /**
+     * Create the parameters from the property_tree.
+     * Because this is directly tied to the specific type. The value/index type
+     * settings are ignored and type_descriptor is for children objects.
+     *
+     * @param config  the property tree for setting
+     * @param context  the registry
+     * @param td_for_child  the type descriptor for children objects. The
+     *                      default will directly from the specific type.
+     *
+     * @return parameters
+     */
     static parameters_type parse(const config::pnode& config,
                                  const config::registry& context,
                                  const config::type_descriptor& td_for_child =