Expand Lock.protect docs

To explain the lock must be instantiated outside the threaded area

doc/Type/Lock.pod6

@@ -37,8 +37,36 @@ Defined as:
 
 Runs C<&code> and makes sure it is only run in one thread at once.
 
-    my $l = Lock.new;
-    $l.protect({ #`( some unsafe operations here ) 1 + 1; });
+Note that the L<Lock> itself needs to be created outside the portion
+of the code that gets threaded and it needs to protect. In the first
+example below, L<Lock> is first created and assigned to C<$lock>,
+which is then used I<inside> the L<Promises|/type/Promise> to protect
+the sensitive code. In the second example, a mistake is made, the
+C<Lock> is created right inside the L<Promise>, so the code ends up
+with a bunch of separate locks, created in a bunch of threads, and
+thus they don't actually protect the code we want to protect.
+
+    # Right: $lock is instantiated outside the portion of the
+    # code that will get threaded and be in need of protection
+    my $lock = Lock.new;
+    await ^20 .map: {
+        start {
+            $lock.protect: {
+                print "Foo";
+                sleep rand;
+                say "Bar";
+            }
+        }
+    }
+
+    # !!! WRONG !!! Lock is created inside threaded area!
+    await ^20 .map: {
+        start {
+            Lock.new.protect: {
+                print "Foo"; sleep rand; say "Bar";
+            }
+        }
+    }
 
 =head2 method lock