Rollup merge of #142162 - RalfJung:unsafe-pinned-get, r=workingjubilee,traviscross

UnsafePinned: update get() docs and signature to allow shared mutation

Follow-up to rust-lang/rust#140638, making `get` consistent with the fact that there's an `UnsafeCell` inside this type now by returning `*mut T` instead of `*const T`.

Cc ``@rust-lang/libs-api``
Tracking issue: rust-lang/rust#125735

Author: workingjubilee