Issue #20226: Major improvements to Argument Clinic.

* You may now specify an expression as the default value for a
  parameter!  Example: "sys.maxsize - 1".  This support is
  intentionally quite limited; you may only use values that
  can be represented as static C values.
* Removed "doc_default", simplified support for "c_default"
  and "py_default".  (I'm not sure we still even need
  "py_default", but I'm leaving it in for now in case a
  use presents itself.)
* Parameter lines support a trailing '\\' as a line
  continuation character, allowing you to break up long lines.
* The argument parsing code generated when supporting optional
  groups now uses PyTuple_GET_SIZE instead of PyTuple_GetSize,
  leading to a 850% speedup in parsing.  (Just kidding, this
  is an unmeasurable difference.)
* A bugfix for the recent regression where the generated
  prototype from pydoc for builtins would be littered with
  unreadable "=<object ...>"" default values for parameters
  that had no default value.
* Converted some asserts into proper failure messages.
* Many doc improvements and fixes.

Doc/c-api/arg.rst

@@ -294,6 +294,8 @@ Other objects
    the object pointer is stored.  If the Python object does not have the required
    type, :exc:`TypeError` is raised.
 
+.. _o_ampersand:
+
 ``O&`` (object) [*converter*, *anything*]
    Convert a Python object to a C variable through a *converter* function.  This
    takes two arguments: the first is a function, the second is the address of a C