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]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Makefile

@@ -460,7 +460,8 @@ export rust_common_flags := --edition=2021 \
 			    -Wclippy::undocumented_unsafe_blocks \
 			    -Wclippy::unnecessary_safety_comment \
 			    -Wclippy::unnecessary_safety_doc \
-			    -Wrustdoc::missing_crate_level_docs
+			    -Wrustdoc::missing_crate_level_docs \
+			    -Wrustdoc::unescaped_backticks
 
 KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) \
 		       $(HOSTCFLAGS) -I $(srctree)/scripts/include

rust/Makefile

@@ -61,7 +61,7 @@ alloc-cfgs = \
 quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $<
       cmd_rustdoc = \
 	OBJTREE=$(abspath $(objtree)) \
-	$(RUSTDOC) $(if $(rustdoc_host),$(rust_common_flags),$(rust_flags)) \
+	$(RUSTDOC) $(filter-out $(skip_flags),$(if $(rustdoc_host),$(rust_common_flags),$(rust_flags))) \
 		$(rustc_target_flags) -L$(objtree)/$(obj) \
 		-Zunstable-options --generate-link-to-definition \
 		--output $(rustdoc_output) \
@@ -98,6 +98,9 @@ rustdoc-macros: private rustc_target_flags = --crate-type proc-macro \
 rustdoc-macros: $(src)/macros/lib.rs FORCE
 	+$(call if_changed,rustdoc)
 
+# Starting with Rust 1.82.0, skipping `-Wrustdoc::unescaped_backticks` should
+# not be needed -- see https://github.com/rust-lang/rust/pull/128307.
+rustdoc-core: private skip_flags = -Wrustdoc::unescaped_backticks
 rustdoc-core: private rustc_target_flags = $(core-cfgs)
 rustdoc-core: $(RUST_LIB_SRC)/core/src/lib.rs FORCE
 	+$(call if_changed,rustdoc)