Document the conditional existence of alloc::sync and alloc::task.

The wording is copied from `std::sync::atomic::AtomicPtr`, with
additional advice on how to `#[cfg]` for it.

library/alloc/src/sync.rs

@@ -3,6 +3,10 @@
 //! Thread-safe reference-counting pointers.
 //!
 //! See the [`Arc<T>`][Arc] documentation for more details.
+//!
+//! **Note**: This module is only available on platforms that support atomic
+//! loads and stores of pointers. This may be detected at compile time using
+//! `#[cfg(target_has_atomic = "ptr")]`.
 
 use core::any::Any;
 use core::borrow;
@@ -82,6 +86,11 @@ macro_rules! acquire {
 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
 /// types.
 ///
+/// **Note**: This type is only available on platforms that support atomic
+/// loads and stores of pointers, which includes all platforms that support
+/// the `std` crate but not all those which only support [`alloc`](crate).
+/// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
+///
 /// ## Thread Safety
 ///
 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference

library/alloc/src/task.rs

@@ -1,5 +1,11 @@
 #![stable(feature = "wake_trait", since = "1.51.0")]
+
 //! Types and Traits for working with asynchronous tasks.
+//!
+//! **Note**: This module is only available on platforms that support atomic
+//! loads and stores of pointers. This may be detected at compile time using
+//! `#[cfg(target_has_atomic = "ptr")]`.
+
 use core::mem::ManuallyDrop;
 use core::task::{RawWaker, RawWakerVTable, Waker};