docs: automarkup.py: Allow automatic cross-reference inside C namespace

docs: automarkup.py: Allow automatic cross-reference inside C namespace

Sphinx 3.1 introduced namespaces for C cross-references. With this,
each C domain type/function declaration is put inside the namespace that
was active at the time of its declaration.

Add support for automatic cross-referencing inside C namespaces by
checking whether the corresponding source file had a C namespace Sphinx
directive, and if so, try cross-referencing inside of it before going to
the global scope.

This assumes there's only one namespace (if any) per rst file.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Link: https://lore.kernel.org/r/20201117021107.214704-1-nfraprado@protonmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>