Make clone unsafe

There are many features of `clone` that may cause memory unsafety when
called. This documents one of them and references `fork()`, which is
already unsafe to call.

src/sched.rs

@@ -95,7 +95,17 @@ mod sched_linux_like {
     /// address need not be the highest address of the region.  Nix will take
     /// care of that requirement.  The user only needs to provide a reference to
     /// a normally allocated buffer.
-    pub fn clone(
+    ///
+    /// # Safety
+    ///
+    /// Because `clone` creates a child process with its stack located in
+    /// `stack` without specifying the size of the stack, special care must be
+    /// taken to ensure that the child process does not overflow the provided
+    /// stack space.
+    ///
+    /// See [`fork`](crate::unistd::fork) for additional safety concerns related
+    /// to executing child processes.
+    pub unsafe fn clone(
         mut cb: CloneCb,
         stack: &mut [u8],
         flags: CloneFlags,