Update comment

src/mmtk.rs

@@ -240,11 +240,22 @@ impl<VM: VMBinding> MMTK<VM> {
 
     /// Prepare an MMTk instance for calling the `fork()` system call.
     ///
-    /// This function makes all MMTk threads (currently including GC worker threads) save their
-    /// contexts and stop.  A subsequent call to `MMTK::after_fork()` will re-spawn the threads
-    /// using the saved contexts.
+    /// The `fork()` system call is available on Linux and some UNIX variants.  When `fork()` is
+    /// called when the process has multiple threads, it will only duplicate the current thread
+    /// into the child process, and the child process can only call async-signal-safe functions,
+    /// notably `exec()`.  For VMs that that use multi-process concurrency, it is imperative that
+    /// when calling `fork()`, only one thread may exist in the process.
     ///
-    /// This function returns when all MMTK threads stopped.
+    /// This function helps VMs that use `fork()` for multi-process concurrency.  It makes all MMTk
+    /// threads (currently including GC worker threads) save their contexts and stop.  This
+    /// function returns when all MMTK threads stopped.  A subsequent call to `MMTK::after_fork()`
+    /// will re-spawn the threads using the saved contexts.
+    ///
+    /// To use thie function, the VM needs to implement [`crate::vm::Collection::spawn_gc_thread`]
+    /// so that the native thread exits after the entry-point functions of GC threads, such as
+    /// [`crate::memory_manager::start_worker`], exits.  Its returned
+    /// [`GCThreadJoinHandle`](crate::vm::gc_thread::GCThreadJoinHandle) must wait for the
+    /// underlying native thread to exit.
     pub fn prepare_to_fork(&'static self) {
         assert!(
             self.state.is_initialized(),