Reword safety guarantee of Pin::static_{ref,mut}.

Co-authored-by: Peter Todd <pete@petertodd.org>

library/core/src/pin.rs

@@ -784,12 +784,12 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
 impl<T: ?Sized> Pin<&'static T> {
     /// Get a pinned reference from a static reference.
     ///
-    /// This is safe, because the `'static` lifetime guarantees the data will
-    /// never be moved.
+    /// This is safe, because `T` is borrowed for the `'static` lifetime, which
+    /// never ends.
     #[unstable(feature = "pin_static_ref", issue = "none")]
     #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
     pub const fn static_ref(r: &'static T) -> Pin<&'static T> {
-        // SAFETY: The 'static lifetime guarantees the data will not be
+        // SAFETY: The 'static borrow guarantees the data will not be
         // moved/invalidated until it gets dropped (which is never).
         unsafe { Pin::new_unchecked(r) }
     }
@@ -798,12 +798,12 @@ impl<T: ?Sized> Pin<&'static T> {
 impl<T: ?Sized> Pin<&'static T> {
     /// Get a pinned mutable reference from a static mutable reference.
     ///
-    /// This is safe, because the `'static` lifetime guarantees the data will
-    /// never be moved.
+    /// This is safe, because `T` is borrowed for the `'static` lifetime, which
+    /// never ends.
     #[unstable(feature = "pin_static_ref", issue = "none")]
     #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
     pub const fn static_mut(r: &'static mut T) -> Pin<&'static mut T> {
-        // SAFETY: The 'static lifetime guarantees the data will not be
+        // SAFETY: The 'static borrow guarantees the data will not be
         // moved/invalidated until it gets dropped (which is never).
         unsafe { Pin::new_unchecked(r) }
     }