add caveats for unlinking

Author: tbrowder

doc/Language/io.pod6

@@ -132,8 +132,8 @@ rename $filea, 'new-foo', :createonly; # fails if 'new-foo' exists
 move $fileb, '/disk2/foo';              # overwrites '/disk2/foo' if it exists
 move $fileb, '/disk2/foo', :createonly; # fails if '/disk2/foo' exists
 
-unlink $filea;    # removes $filea
-$fileb.IO.unlink; # removes $fileb
+unlink $filea;    # removes $filea if it exists, fails with incorrect permissions
+$fileb.IO.unlink; # removes $fileb if it exists, fails with incorrect permissions
 
 =end code