rust: task: add missing Markdown code spans and intra-doc links

Add missing Markdown code spans and also convert them into intra-doc
links.

This was found using the Clippy `doc_markdown` lint, which we may want
to enable.

Fixes: e0020ba6cb ("rust: add PidNamespace")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250324210359.1199574-10-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/task.rs

@@ -158,10 +158,10 @@ impl Task {
         }
     }
 
-    /// Returns a PidNamespace reference for the currently executing task's/thread's pid namespace.
+    /// Returns a [`PidNamespace`] reference for the currently executing task's/thread's pid namespace.
     ///
     /// This function can be used to create an unbounded lifetime by e.g., storing the returned
-    /// PidNamespace in a global variable which would be a bug. So the recommended way to get the
+    /// [`PidNamespace`] in a global variable which would be a bug. So the recommended way to get the
     /// current task's/thread's pid namespace is to use the [`current_pid_ns`] macro because it is
     /// safe.
     ///