[3.8] bpo-38387: Formally document PyDoc_STRVAR and PyDoc_STR macros (GH-16607) ...

[3.8] bpo-38387: Formally document PyDoc_STRVAR and PyDoc_STR macros (GH-16607) (GH-19727)

Adds a short description of `PyDoc_STRVAR` and `PyDoc_STR` to "Useful macros" section of C-API docs.

Currently, there is [one lone mention](https://docs.python.org/3/c-api/module.html?highlight=pydoc_strvarGH-c.PyModuleDef) in the C-API reference, despite the fact that `PyDoc_STRVAR` is ubiquitous to `Modules/`.

Additionally, this properly uses `c:macro` within `Doc/c-api/module.rst` to link.
(cherry picked from commit b54e46c)

Authored-by: Brad Solomon <brad.solomon.1124@gmail.com>