postPut in new array API

Call when appropriate (putFrom(), copyIn(), and replace()).
Not called by swap(), take(), reuse(), or shareData().
Users of the second set of methods are expected to call
one of the methods in the first set, or call postPut() directly.

Document when postPut is (not) called.

testApp/pv/testPVScalarArray.cpp

@@ -108,6 +108,7 @@ static void testBasic()
     testOk1(!arr1->viewUnsafe().unique());
 
     arr2->swap(data);
+    arr2->postPut();
 
     testOk1(arr2->getLength()==0);
     testOk1(data.size()==arr1->getLength());