Document the use of available_parallelism

(cherry picked from commit 24723fa)

Author: cuviper

rayon-core/src/lib.rs

@@ -552,9 +552,8 @@ impl<S> ThreadPoolBuilder<S> {
     /// may change in the future, if you wish to rely on a fixed
     /// number of threads, you should use this function to specify
     /// that number. To reproduce the current default behavior, you
-    /// may wish to use the [`num_cpus`
-    /// crate](https://crates.io/crates/num_cpus) to query the number
-    /// of CPUs dynamically.
+    /// may wish to use [`std::thread::available_parallelism`]
+    /// to query the number of CPUs dynamically.
     ///
     /// **Old environment variable:** `RAYON_NUM_THREADS` is a one-to-one
     /// replacement of the now deprecated `RAYON_RS_NUM_CPUS` environment