std: improve documentation for get_mut() methods regarding forgotten guards

library/core/src/cell.rs

@@ -1163,7 +1163,9 @@ impl<T: ?Sized> RefCell<T> {
     /// Since this method borrows `RefCell` mutably, it is statically guaranteed
     /// that no borrows to the underlying data exist. The dynamic checks inherent
     /// in [`borrow_mut`] and most other methods of `RefCell` are therefore
-    /// unnecessary.
+    /// unnecessary. Note that this method does not reset the borrowing state if borrows were previously leaked
+    /// (e.g., via [`forget()`] on a [`Ref`] or [`RefMut`]). For that purpose,
+    /// consider using the unstable [`undo_leak`] method.
     ///
     /// This method can only be called if `RefCell` can be mutably borrowed,
     /// which in general is only the case directly after the `RefCell` has
@@ -1174,6 +1176,8 @@ impl<T: ?Sized> RefCell<T> {
     /// Use [`borrow_mut`] to get mutable access to the underlying data then.
     ///
     /// [`borrow_mut`]: RefCell::borrow_mut()
+    /// [`forget()`]: mem::forget
+    /// [`undo_leak`]: RefCell::undo_leak()
     ///
     /// # Examples
     ///

library/std/src/sync/poison/mutex.rs

@@ -582,7 +582,9 @@ impl<T: ?Sized> Mutex<T> {
     /// Returns a mutable reference to the underlying data.
     ///
     /// Since this call borrows the `Mutex` mutably, no actual locking needs to
-    /// take place -- the mutable borrow statically guarantees no locks exist.
+    /// take place -- the mutable borrow statically guarantees no new locks can be acquired
+    /// while this reference exists. Note that this method does not clear any previous abandoned locks
+    /// (e.g., via [`forget()`] on a [`MutexGuard`]).
     ///
     /// # Errors
     ///
@@ -599,6 +601,8 @@ impl<T: ?Sized> Mutex<T> {
     /// *mutex.get_mut().unwrap() = 10;
     /// assert_eq!(*mutex.lock().unwrap(), 10);
     /// ```
+    ///
+    /// [`forget()`]: mem::forget
     #[stable(feature = "mutex_get_mut", since = "1.6.0")]
     pub fn get_mut(&mut self) -> LockResult<&mut T> {
         let data = self.data.get_mut();

library/std/src/sync/poison/rwlock.rs

@@ -608,7 +608,9 @@ impl<T: ?Sized> RwLock<T> {
     /// Returns a mutable reference to the underlying data.
     ///
     /// Since this call borrows the `RwLock` mutably, no actual locking needs to
-    /// take place -- the mutable borrow statically guarantees no locks exist.
+    /// take place -- the mutable borrow statically guarantees no new locks can be acquired
+    /// while this reference exists. Note that this method does not clear any previously abandoned locks
+    /// (e.g., via [`forget()`] on a [`RwLockReadGuard`] or [`RwLockWriteGuard`]).
     ///
     /// # Errors
     ///