[BUG] Descriptions of static members are not printed via help command

Issue description

The documentation gives an example about providing description for a definition which can be seen by help.

Static members of a class can be accessed via def_readonly_static and def_readwrite_static.

The problem is that all descriptions are visible with help command except that of static members.

Reproducible example code

include/ClassTest.hpp

#ifndef CLASSTEST_HPP
#define CLASSTEST_HPP

#include <string>

struct ClassTest
{
    static std::string static_data;
    static std::string print_static_data();

    std::string _data;
    ClassTest(const std::string& data);
    std::string print_data() const;
    std::string& change_data(std::string& data);
};

#endif  // CLASSTEST_HPP

---

cpp2py.cpp

#include "ClassTest.hpp"

#include <pybind11/pybind11.h>
namespace py = pybind11;

PYBIND11_MODULE(class_test, m)
{
    m.doc() = "pybind11 example class";

    py::class_<ClassTest>(m, "ClassTest", "the example class")
        .def_readonly ("data_readonly", &ClassTest::_data, "read-only data" )
        .def_readwrite("data"         , &ClassTest::_data, "read-write data")
        .def_readonly_static ("static_data_readonly", &ClassTest::static_data, "static read-only data" )
        .def_readwrite_static("static_data"         , &ClassTest::static_data, "static read-write data")
        .def_static("print_static_data", &ClassTest::print_static_data,
            "static method that prints and returns a copy of static_data")
        .def(py::init<const std::string &>(), "initialize data")
        .def("print_data" , &ClassTest::print_data , "const method that prints data")
        .def("change_data", &ClassTest::change_data, "modifier method that returns reference to data");
}

---

test_class.py

import class_test
help(class_test)

Output:

Help on module class_test:

NAME
    class_test - pybind11 example class

CLASSES
    pybind11_builtins.pybind11_object(builtins.object)
        ClassTest
    
    class ClassTest(pybind11_builtins.pybind11_object)
     |  the example class
     |  
     |  Method resolution order:
     |      ClassTest
     |      pybind11_builtins.pybind11_object
     |      builtins.object
     |  
     |  Methods defined here:
     |  
     |  __init__(...)
     |      __init__(self: class_test.ClassTest, arg0: str) -> None
     |      
     |      initialize data
     |  
     |  change_data(...)
     |      change_data(self: class_test.ClassTest, arg0: str) -> str
     |      
     |      modifier method that returns reference to data
     |  
     |  print_data(...)
     |      print_data(self: class_test.ClassTest) -> str
     |      
     |      const method that prints data
     |  
     |  ----------------------------------------------------------------------
     |  Static methods defined here:
     |  
     |  print_static_data(...) from builtins.PyCapsule
     |      print_static_data() -> str
     |      
     |      static method that prints and returns a copy of static_data
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors defined here:
     |  
     |  data
     |      read-write data
     |  
     |  data_readonly
     |      read-only data
     |  
     |  static_data
     |  
     |  static_data_readonly
     |  
     |  ----------------------------------------------------------------------
     |  Methods inherited from pybind11_builtins.pybind11_object:
     |  
     |  __new__(*args, **kwargs) from pybind11_builtins.pybind11_type
     |      Create and return a new object.  See help(type) for accurate signature.

FILE
    pybind11_example\class_test.cp36-win_amd64.pyd

Note that descriptions for static_data and static_data_readonly are missing.

pybind11 version: v2.6.2

The other files

src/ClassTest.cpp

#include <iostream>
#include "ClassTest.hpp"

std::string ClassTest::static_data = "static_data";

std::string ClassTest::print_static_data()
{
    std::cout << static_data << std::endl;
    return static_data;
}

ClassTest::ClassTest(const std::string& data) : _data(data) {}

std::string ClassTest::print_data() const
{
    std::cout << _data << std::endl;
    return _data;
}

std::string& ClassTest::change_data(std::string& data)
{
    _data = data;
    return _data;
}

---

CMakeLists.txt

cmake_minimum_required(VERSION 3.4)
project(pybind11example)
set(CMAKE_CXX_STANDARD 11)
include_directories(include)
add_subdirectory(pybind11)
pybind11_add_module(class_test cpp2py.cpp src/ClassTest.cpp)
target_compile_definitions(class_test PRIVATE)

---

setup.py

from setuptools import setup
setup(name='class_test')

Reproduce the missing descriptions with the shown cpp2py.cpp binding and test_class.py help call. Start by tracing def_readonly_static and def_readwrite_static, then compare their generated help output with the instance fields and static method; done means both static member descriptions appear in help.