Add Pin::as_deref_mut to stabilized APIs

Co-authored-by: Travis Cross <tc@traviscross.com>

Author: BoxyUwU

posts/2025-01-09-Rust-1.84.0.md

@@ -143,6 +143,7 @@ For more details, see the standard library [documentation on provenance](https:/
 - [`core::ptr::without_provenance_mut`](https://doc.rust-lang.org/stable/core/ptr/fn.without_provenance_mut.html)
 - [`core::ptr::dangling`](https://doc.rust-lang.org/stable/core/ptr/fn.dangling.html)
 - [`core::ptr::dangling_mut`](https://doc.rust-lang.org/stable/core/ptr/fn.dangling_mut.html)
+- [`Pin::as_deref_mut`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.as_deref_mut)
 
 These APIs are now stable in const contexts