rust: clean and enable Clippy `doc_markdown` lint

This is how it would look like -- I am not sure how many false positives we will get, so I marked the last commit as RFC and perhaps we just want this lint as an opt-in in e.g. W=2.

However, so far, we only got one ("KUnit"), plus a couple others that are in C side comments that were copied, and thus someone may want to keep them exactly in sync. So it seems pretty OK so far.

Another question is whether we want to commit to something like the global list in .clippy.toml.

Anyway, please take a look -- the other commits should probably be picked even if we do not enable the lint.

Thanks!

Miguel Ojeda (10):
drm/panic: add missing space
drm/panic: add missing Markdown code span
rust: alloc: add missing Markdown code spans
rust: alloc: add missing Markdown code span
rust: dma: add missing Markdown code span
rust: of: add missing Markdown code span
rust: pci: fix docs related to missing Markdown code spans
rust: platform: fix docs related to missing Markdown code spans
rust: task: add missing Markdown code spans and intra-doc links
rust: kbuild: enable doc_markdown Clippy lint

 .clippy.toml                        |  4 ++++
 Makefile                            |  1 +
 drivers/gpu/drm/drm_panic_qr.rs     |  4 ++--
 rust/kernel/alloc.rs                |  4 ++--
 rust/kernel/alloc/allocator_test.rs |  2 +-
 rust/kernel/dma.rs                  |  2 +-
 rust/kernel/of.rs                   |  2 +-
 rust/kernel/pci.rs                  | 15 +++++++++------
 rust/kernel/platform.rs             |  9 +++++----
 rust/kernel/task.rs                 |  4 ++--
 10 files changed, 28 insertions(+), 19 deletions(-)

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2.49.0