rust: make section names plural

Clean Rust documentation section headers to use plural names.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: #1110
Signed-off-by: Patrick Miller <paddymills@proton.me>
Link: https://lore.kernel.org/r/20241002022749.390836-1-paddymills@proton.me
[ Removed the `init` one that doesn't apply anymore and
  reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: paddymills

rust/kernel/list/arc.rs

@@ -464,7 +464,7 @@ where
 
 /// A utility for tracking whether a [`ListArc`] exists using an atomic.
 ///
-/// # Invariant
+/// # Invariants
 ///
 /// If the boolean is `false`, then there is no [`ListArc`] for this value.
 #[repr(transparent)]

rust/kernel/sync/arc.rs

@@ -492,7 +492,7 @@ impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
 /// There are no mutable references to the underlying [`Arc`], and it remains valid for the
 /// lifetime of the [`ArcBorrow`] instance.
 ///
-/// # Example
+/// # Examples
 ///
 /// ```
 /// use kernel::sync::{Arc, ArcBorrow};

rust/macros/lib.rs

@@ -263,7 +263,7 @@ pub fn concat_idents(ts: TokenStream) -> TokenStream {
 /// literals (lifetimes and documentation strings are not supported). There is a difference in
 /// supported modifiers as well.
 ///
-/// # Example
+/// # Examples
 ///
 /// ```
 /// # const binder_driver_return_protocol_BR_OK: u32 = 0;