Clarify &mut-methods' docs on sync::OnceLock

Author: lukaslueg

library/std/src/sync/once_lock.rs

@@ -159,8 +159,11 @@ impl<T> OnceLock<T> {
 
  /// Gets the mutable reference to the underlying value.
  ///
- /// Returns `None` if the cell is uninitialized, or being initialized.
- /// This method never blocks.
+ /// Returns `None` if the cell is uninitialized.
+ ///
+ /// This method never blocks. Since it borrows the `OnceLock` mutably,
+ /// it is statically guaranteed that no active borrows to the `OnceLock`
+ /// exist, including from other threads.
  #[inline]
  #[stable(feature = "once_cell", since = "1.70.0")]
  pub fn get_mut(&mut self) -> Option<&mut T> {
@@ -315,7 +318,9 @@ impl<T> OnceLock<T> {
  /// Gets the mutable reference of the contents of the cell, initializing
  /// it to `f()` if the cell was uninitialized.
  ///
- /// This method never blocks.
+ /// This method never blocks. Since it borrows the `OnceLock` mutably,
+ /// it is statically guaranteed that no active borrows to the `OnceLock`
+ /// exist, including from other threads.
  ///
  /// # Panics
  ///
@@ -405,7 +410,9 @@ impl<T> OnceLock<T> {
  /// it to `f()` if the cell was uninitialized. If the cell was uninitialized
  /// and `f()` failed, an error is returned.
  ///
- /// This method never blocks.
+ /// This method never blocks. Since it borrows the `OnceLock` mutably,
+ /// it is statically guaranteed that no active borrows to the `OnceLock`
+ /// exist, including from other threads.
  ///
  /// # Panics
  ///
@@ -469,7 +476,8 @@ impl<T> OnceLock<T> {
  ///
  /// Has no effect and returns `None` if the `OnceLock` was uninitialized.
  ///
- /// Safety is guaranteed by requiring a mutable reference.
+ /// Since this method borrows the `OnceLock` mutably, it is statically guaranteed that
+ /// no active borrows to the `OnceLock` exist, including from other threads.
  ///
  /// # Examples
  ///