rust: use consistent backtick formatting for NULL in docs

rust: use consistent backtick formatting for NULL in docs

Some doc comments use `NULL` while others use plain NULL.  Make it
consistent by adding backticks everywhere, matching the majority of
existing usage.

Signed-off-by: Peter Novak <seimun018r@gmail.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Acked-by: David Gow <davidgow@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20251130211233.367946-1-seimun018r@gmail.com
[ Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>