tools/docs: sphinx-build-wrapper: only generate rust docs when requested

tools/docs: sphinx-build-wrapper: only generate rust docs when requested

When the user explicitly specifies SPHINXDIRS to build a specific
subdirectory it is unexpected that the rust docs are also generated.
Especially as their generation may dominate the execution time.

Only generate the rust docs when they are part of the SPHINXDIRS
requested by the user. 'rust/rustdocs' is not considered, as it is
not a valid SPHINXDIRS anyways.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Tested-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260118-docs-spurious-rust-v1-4-998e14b9ed9e@weissschuh.net>