rust: pin-init: improve safety documentation for impl<T> [Pin]Init<T> for T

The inner SAFETY comments were missing since commit 5cfe7bef6751 ("rust:
enable `clippy::undocumented_unsafe_blocks` lint").

Also rework the implementation of `__pinned_init` to better justify the
SAFETY comment.

Link: df925b2e27
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/all/20250529081027.297648-1-lossin@kernel.org
Signed-off-by: Benno Lossin <lossin@kernel.org>

rust/pin-init/src/lib.rs

@@ -1390,20 +1390,22 @@ where
     unsafe { pin_init_from_closure(init) }
 }
 
-// SAFETY: Every type can be initialized by-value.
+// SAFETY: the `__init` function always returns `Ok(())` and initializes every field of `slot`.
 unsafe impl<T, E> Init<T, E> for T {
     unsafe fn __init(self, slot: *mut T) -> Result<(), E> {
-        // SAFETY: TODO.
+        // SAFETY: `slot` is valid for writes by the safety requirements of this function.
         unsafe { slot.write(self) };
         Ok(())
     }
 }
 
-// SAFETY: Every type can be initialized by-value. `__pinned_init` calls `__init`.
+// SAFETY: the `__pinned_init` function always returns `Ok(())` and initializes every field of
+// `slot`. Additionally, all pinning invariants of `T` are upheld.
 unsafe impl<T, E> PinInit<T, E> for T {
     unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
-        // SAFETY: TODO.
-        unsafe { self.__init(slot) }
+        // SAFETY: `slot` is valid for writes by the safety requirements of this function.
+        unsafe { slot.write(self) };
+        Ok(())
     }
 }