Docs: Argument Clinic: Improve 'How to write a custom converter' (#107328)

- Omit unneccesary wording and sentences
- Don't mention implementation details (no digression, explanation)

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

Doc/howto/clinic.rst

@@ -1343,35 +1343,37 @@ state.  Example from the ``setattro`` slot method in
 See also :pep:`573`.
 
 
+.. _clinic-howto-custom-converter:
+
 How to write a custom converter
 -------------------------------
 
-As we hinted at in the previous section... you can write your own converters!
-A converter is simply a Python class that inherits from :py:class:`!CConverter`.
-The main purpose of a custom converter is if you have a parameter using
-the ``O&`` format unit—parsing this parameter means calling
+A converter is a Python class that inherits from :py:class:`!CConverter`.
+The main purpose of a custom converter, is for parameters parsed with
+the ``O&`` format unit --- parsing such a parameter means calling
 a :c:func:`PyArg_ParseTuple` "converter function".
 
-Your converter class should be named ``*something*_converter``.
-If the name follows this convention, then your converter class
-will be automatically registered with Argument Clinic; its name
-will be the name of your class with the ``_converter`` suffix
-stripped off.  (This is accomplished with a metaclass.)
+Your converter class should be named :samp:`{ConverterName}_converter`.
+By following this convention, your converter class will be automatically
+registered with Argument Clinic, with its *converter name* being the name of
+your converter class with the ``_converter`` suffix stripped off.
 
-You shouldn't subclass :py:meth:`!CConverter.__init__`.  Instead, you should
-write a :py:meth:`!converter_init` function. :py:meth:`!converter_init`
-always accepts a *self* parameter; after that, all additional
-parameters *must* be keyword-only.  Any arguments passed in to
-the converter in Argument Clinic will be passed along to your
-:py:meth:`!converter_init`.
-
-There are some additional members of :py:class:`!CConverter` you may wish
-to specify in your subclass.  Here's the current list:
+Instead of subclassing :py:meth:`!CConverter.__init__`,
+write a :py:meth:`!converter_init` method.
+Apart for the *self* parameter, all additional :py:meth:`!converter_init`
+parameters **must** be keyword-only.
+Any arguments passed to the converter in Argument Clinic
+will be passed along to your :py:meth:`!converter_init` method.
+See :py:class:`!CConverter` for a list of members you may wish to specify in
+your subclass.
 
 .. module:: clinic
 
 .. class:: CConverter
 
+   The base class for all converters.
+   See :ref:`clinic-howto-custom-converter` for how to subclass this class.
+
    .. attribute:: type
 
       The C type to use for this variable.
@@ -1436,16 +1438,16 @@ Here's the simplest example of a custom converter, from :source:`Modules/zlibmod
     [python start generated code]*/
     /*[python end generated code: output=da39a3ee5e6b4b0d input=35521e4e733823c7]*/
 
-This block adds a converter to Argument Clinic named ``ssize_t``.  Parameters
-declared as ``ssize_t`` will be declared as type :c:type:`Py_ssize_t`, and will
-be parsed by the ``'O&'`` format unit, which will call the
-``ssize_t_converter`` converter function.  ``ssize_t`` variables
-automatically support default values.
+This block adds a converter named ``ssize_t`` to Argument Clinic.
+Parameters declared as ``ssize_t`` will be declared with type :c:type:`Py_ssize_t`,
+and will be parsed by the ``'O&'`` format unit,
+which will call the :c:func:`!ssize_t_converter` converter C function.
+``ssize_t`` variables automatically support default values.
 
 More sophisticated custom converters can insert custom C code to
 handle initialization and cleanup.
 You can see more examples of custom converters in the CPython
-source tree; grep the C files for the string :py:class:`!CConverter`.
+source tree; grep the C files for the string ``CConverter``.
 
 
 How to write a custom return converter