docs: Makefile: update SPHINXDIRS documentation

Since the beginning, SPHINXDIRS was meant to be used by any
subdirectory inside Documentation/ that contains a file named
index.rst on it. The typical usecase for SPHINXDIRS is help
building subsystem-specific documentation, without needing to
wait for the entire building (with can take 3 minutes with
Sphinx 8.x and above, and a lot more with older versions).

Yet, the documentation for such feature was written back in
2016, where almost all index.rst files were at the first
level (Documentation/*/index.rst).

Update the documentation to reflect the way it works.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <683469813350214da122c258063dd71803ff700b.1763031632.git.mchehab+huawei@kernel.org>

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