Stubgen doesn't look at C type's docstring when inferring __init__'s type

Bug Report

Given a class with a docstring similar to "ClassName(initparam: str)" or "ClassName(initparam: str) -> ClassName" or "(initparam: str)" or "__init__(initparam: str) -> None" the stub generation via runtime introspection will not digest this information.

This is relevant, because the CPython API doesn't allow setting the __init__ method's docstring, like other members' or type objects', making automatic annotation of constructors impossible.

To Reproduce

You have to build a C extension, but I can provide modules if needed for testing.

PyTypeObject TestType_ = {                                                  \
    PyVarObject_HEAD_INIT(NULL, 0)                                          \
    /* tp_name            */   "module.Test",                               \
    /* tp_basicsize       */   sizeof(t_test),                              \
    /* tp_itemsize        */   0,                                           \
    /* tp_dealloc         */   (destructor)dealloc,                         \
    /* tp_print           */   0,                                           \
    /* tp_getattr         */   0,                                           \
    /* tp_setattr         */   0,                                           \
    /* tp_compare         */   0,                                           \
    /* tp_repr            */   0,                                           \
    /* tp_as_number       */   0,                                           \
    /* tp_as_sequence     */   0,                                           \
    /* tp_as_mapping      */   0,                                           \
    /* tp_hash            */   0,                                           \
    /* tp_call            */   0,                                           \
    /* tp_str             */   0,                                           \
    /* tp_getattro        */   0,                                           \
    /* tp_setattro        */   0,                                           \
    /* tp_as_buffer       */   0,                                           \
    /* tp_flags           */   Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,    \
    /* tp_doc             */   PyDoc_STR("Test(param: int)"),               \
    /* tp_traverse        */   0,                                           \
    /* tp_clear           */   0,                                           \
    /* tp_richcompare     */   0,                                           \
    /* tp_weaklistoffset  */   0,                                           \
    /* tp_iter            */   0,                                           \
    /* tp_iternext        */   0,                                           \
    /* tp_methods         */   t_test_methods,                              \
    /* tp_members         */   0,                                           \
    /* tp_getset          */   0,                                           \
    /* tp_base            */   0,                                           \
    /* tp_dict            */   0,                                           \
    /* tp_descr_get       */   0,                                           \
    /* tp_descr_set       */   0,                                           \
    /* tp_dictoffset      */   0,                                           \
    /* tp_init            */   (initproc)t_type_init,                       \
    /* tp_alloc           */   0,                                           \
    /* tp_new             */   0,                                           \
};

Expected Behavior

Stubgen should generate the lines in .pyi:

class Test():
    def __init__ (self, param: int) -> None: ...
    [other methods]

Actual Behavior

Stubgen instead generates:

class Test():
    def init(self, *args, **kwargs) -> None
    [other methods]

Your Environment

• Mypy version used: 0.990
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): Defaults/None
• Python version used: 3.10

Begin at stubgen's runtime-introspection entry point and inspect how class docstrings and init signatures are handled. Reproduce the C-extension case described in the issue, then add coverage for the listed docstring forms. Done means generated .pyi output uses the constructor parameters from the type docstring instead of generic arguments.