Mention that core::intrinsics::transmute is available at core::mem::t…

…ransmute.

In #[no_std] environments, std::mem::transmute is unavailable. Searching for transmute in libcore only pulls up core::intrinsics::transmute, which is behind the (unstable) core_intrinsics feature flag. Users wishing to use transmute in #[no_std] environments typically should use core::mem::transmute instead, as it is stable. This documentation makes core::mem::transmute discoverable.

src/libcore/intrinsics.rs

@@ -729,6 +729,10 @@ extern "rust-intrinsic" {
     /// cause [undefined behavior][ub] with this function. `transmute` should be
     /// the absolute last resort.
     ///
+    /// `transmute` is re-exported by [core::mem](../mem/index.html) as
+    /// `core::mem::transmute`, which may be used without the `core_intrinsics`
+    /// feature flag.
+    ///
     /// The [nomicon](../../nomicon/transmutes.html) has additional
     /// documentation.
     ///