Be clear that NativeCall'ing fork() won't go well

doc/Language/5to6-perlfunc.pod6

@@ -475,9 +475,18 @@ Currently unimplemented.
 
 =item fork
 
-Not implemented as a built in function, but available through the
-C<NativeCall> interface. E. g. C<use NativeCall; sub fork returns int32 is
-native { * }; say fork;>.
+There is no built-in `fork` function. While it's possible to call it using
+NativeCall, it's highly unlikely that the resulting process will be usable.
+
+Perl 6 provides extensive support for, and internally uses, threads. However,
+`fork` only clones the thread that called `fork`, resulting in a process that
+will be missing its other threads, which will have been in unknown states and
+probably holding locks. Even if a Perl 6 program doesn't knowingly start any
+threads, the compiler may create some of its own in the process of precompilation,
+and the VMs that Perl 6 runs on also create their own internal worker threads for
+doing things like optimization and GC in the background. Thus, the presence of
+threads is pretty much assured, and there's no reasonable way to make `fork`
+reliably work in this case.
 
 =head2 formats