scripts: test_doc_build.py: make the script smarter

scripts: test_doc_build.py: make the script smarter

Most of the time, testing the full range of supported Sphinx
version is a waste of time and resources. Instead, the best is
to focus at the versions that are actually shipped by major
distros.

For it to work properly, we need to adjust the requirements for
them to start from first patch for each distro after the
minimal supported one. The requirements were re-adjusted to
avoid build breakages related to version incompatibilities.
Such builds were tested with:

	./scripts/test_doc_build.py -m -a "SPHINXOPTS=-j8" "SPHINXDIRS=networking netlink/specs" --full

Change the logic to pick by default only such versions, adding
another parameter to do a comprehensive test.

While here, improve the script documentation to make it easier
to be used.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/a2b9b7775a185766643ea4b82b558de25b61d6c7.1750571906.git.mchehab+huawei@kernel.org