bpo-40204, doc: Fix syntax of C variables (GH-21846)

For example, fix the following Sphinx 3 errors:

Doc/c-api/buffer.rst:102: WARNING: Error in declarator or parameters
Invalid C declaration: Expected identifier in nested name. [error at 5]
  void \*obj
  -----^

Doc/c-api/arg.rst:130: WARNING: Unparseable C cross-reference: 'PyObject*'
Invalid C declaration: Expected end of definition. [error at 8]
  PyObject*
  --------^

The modified documentation is compatible with Sphinx 2 and Sphinx 3.

1 files changed, 1 insertions, 1 deletions

diff --git a/Doc/c-api/capsule.rst b/Doc/c-api/capsule.rst
index 78e2114..5eb313c 100644
--- a/Doc/c-api/capsule.rst
+++ b/Doc/c-api/capsule.rst
@@ -15,7 +15,7 @@ Refer to :ref:`using-capsules` for more information on using these objects.
 .. c:type:: PyCapsule
 
    This subtype of :c:type:`PyObject` represents an opaque value, useful for C
-   extension modules who need to pass an opaque value (as a :c:type:`void\*`
+   extension modules who need to pass an opaque value (as a :c:type:`void*`
    pointer) through Python code to other C code.  It is often used to make a C
    function pointer defined in one module available to other modules, so the
    regular import mechanism can be used to access C APIs defined in dynamically