rust: enable rustdoc::unescaped_backticks lint

In Rust 1.71.0, `rustdoc` added the `unescaped_backticks` lint, which
detects what are typically typos in Markdown formatting regarding inline
code [1], e.g. from the Rust standard library:

    /// ... to `deref`/`deref_mut`` must ...

    /// ... use [`from_mut`]`. Specifically, ...

It does not seem to have almost any false positives, from the experience
of enabling it in the Rust standard library [2], which will be checked
starting with Rust 1.82.0. The maintainers also confirmed it is ready
to be used.

Thus enable it.

Link: https://doc.rust-lang.org/rustdoc/lints.html#unescaped_backticks [1]
Link: rust-lang/rust#128307 [2]
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-9-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
(cherry picked from commit bef83245f5ed434932aaf07f890142b576dc5d85)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Cargo.toml

@@ -31,3 +31,6 @@ prettyplease = { version = "0.2", features = ["verbatim"] }
 
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(NO_UI_TESTS)', 'cfg(NO_ALLOC_FAIL_TESTS)'] }
+
+[lints.rustdoc]
+unescaped_backticks = "deny"