rust: driver: fix broken intra-doc links to example driver types

The `auxiliary` and `pci` modules are conditional on
`CONFIG_AUXILIARY_BUS` and `CONFIG_PCI` respectively. When these are
disabled, the intra-doc links to `auxiliary::Driver` and `pci::Driver`
break, causing rustdoc warnings (or errors with `-D warnings`).

error: unresolved link to `kernel::auxiliary::Driver`
  --> rust/kernel/driver.rs:82:28
   |
82 | //! [`auxiliary::Driver`]: kernel::auxiliary::Driver
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `auxiliary` in module `kernel`

Fix this by making the documentation for these examples conditional on
the corresponding configuration options.

Fixes: 970a7c6 ("driver: rust: expand documentation for driver infrastructure")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reported-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Closes: https://lore.kernel.org/rust-for-linux/20251209.151817.744108529426448097.fujita.tomonori@gmail.com/
Link: https://patch.msgid.link/20251227-driver-types-v1-1-1916154fbe5e@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Darksonn

rust/kernel/driver.rs

@@ -33,7 +33,14 @@
 //! }
 //! ```
 //!
-//! For specific examples see [`auxiliary::Driver`], [`pci::Driver`] and [`platform::Driver`].
+//! For specific examples see:
+//!
+//! * [`platform::Driver`](kernel::platform::Driver)
+#![cfg_attr(
+    CONFIG_AUXILIARY_BUS,
+    doc = "* [`auxiliary::Driver`](kernel::auxiliary::Driver)"
+)]
+#![cfg_attr(CONFIG_PCI, doc = "* [`pci::Driver`](kernel::pci::Driver)")]
 //!
 //! The `probe()` callback should return a `impl PinInit<Self, Error>`, i.e. the driver's private
 //! data. The bus abstraction should store the pointer in the corresponding bus device. The generic
@@ -79,16 +86,13 @@
 //!
 //! For this purpose the generic infrastructure in [`device_id`] should be used.
 //!
-//! [`auxiliary::Driver`]: kernel::auxiliary::Driver
 //! [`Core`]: device::Core
 //! [`Device`]: device::Device
 //! [`Device<Core>`]: device::Device<device::Core>
 //! [`Device<CoreInternal>`]: device::Device<device::CoreInternal>
 //! [`DeviceContext`]: device::DeviceContext
 //! [`device_id`]: kernel::device_id
 //! [`module_driver`]: kernel::module_driver
-//! [`pci::Driver`]: kernel::pci::Driver
-//! [`platform::Driver`]: kernel::platform::Driver
 
 use crate::error::{Error, Result};
 use crate::{acpi, device, of, str::CStr, try_pin_init, types::Opaque, ThisModule};