rust: print: use explicit link in documentation

The future `rustdoc` in the Rust 1.73.0 upgrade requires an explicit
link for `pr_info!`:

    error: unresolved link to `pr_info`
       --> rust/kernel/print.rs:395:63
        |
    395 | /// Use only when continuing a previous `pr_*!` macro (e.g. [`pr_info!`]).
        |                                                               ^^^^^^^^ no item named `pr_info` in scope
        |
        = note: `macro_rules` named `pr_info` exists in this crate, but it is not in scope at this link's location
        = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`

Thus do so to avoid a broken link while upgrading.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Finn Behrens <me@kloenk.dev>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20231005210556.466856-3-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/print.rs

@@ -399,6 +399,7 @@ macro_rules! pr_debug (
 /// Mimics the interface of [`std::print!`]. See [`core::fmt`] and
 /// `alloc::format!` for information about the formatting syntax.
 ///
+/// [`pr_info!`]: crate::pr_info!
 /// [`pr_cont`]: https://www.kernel.org/doc/html/latest/core-api/printk-basics.html#c.pr_cont
 /// [`std::print!`]: https://doc.rust-lang.org/std/macro.print.html
 ///