bpo-38387: Formally document PyDoc_STRVAR and PyDoc_STR macros (GH-16607)

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_strvar#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.