Doc update

doc/ref/corelib/json/allocators.md

@@ -10,8 +10,63 @@ or a <a href=https://en.cppreference.com/w/cpp/memory/scoped_allocator_adaptor>s
 Non-propagating stateful allocators, such as the [Boost.Interprocess allocators](https://www.boost.org/doc/libs/1_82_0/doc/html/interprocess/allocators_containers.html#interprocess.allocators_containers.allocator_introduction),
 must be wrapped by a [std::scoped_allocator_adaptor](https://en.cppreference.com/w/cpp/memory/scoped_allocator_adaptor).
 
-Every constructor has a version that accepts an allocator argument. 
+Every constructor has a version that accepts an allocator argument.
 The allocator is used to allocate memory for a long string, byte string, array, or object,
-and it is retained in the long string, byte string, array, or object itself.   
+and it is retained in the long string, byte string, array, or object itself.
 For other data members the allocator argument is ignored. 
 
+#### Move construction
+
+`basic_json` move construction 
+
+```
+Json j1(std::move(j));
+```
+
+copies the pointer or scalar value in `j` to `j1`, and changes the data member in `j` to `null`. For example: 
+
+```
+char buffer[1024] = {};
+std::pmr::monotonic_buffer_resource pool{ std::data(buffer), std::size(buffer) };
+std::pmr::polymorphic_allocator<char> alloc(&pool);
+
+jsoncons::pmr::json j{ "String too long for short string", alloc};
+assert(j.is_string());
+assert(alloc == j.get_allocator());
+
+jsoncons::pmr::json j1(std::move(j));
+assert(j1.is_string());
+assert(alloc == j1.get_allocator());
+assert(j.is_null());
+```
+
+#### Move assignment
+
+If `j` and `j1` both hold pointers, `basic_json` move assignment 
+
+```
+j1 = std::move(j);
+```
+
+swaps the two pointers. If `j` holds a pointer and `j1` holds a non-pointer value,
+move assignment copies `j`'s pointer to `j1` and changes `j`'s value to null.  
+If `j` holds a non-pointer and `j1` holds a pointer value,
+move assignment copies `j`'s value to `j1` and leaves `j` alone. For example:
+
+```cpp
+char buffer[1024];
+std::pmr::monotonic_buffer_resource pool{ std::data(buffer), std::size(buffer) };
+std::pmr::polymorphic_allocator<char> alloc(&pool);
+
+jsoncons::pmr::json j{ "String too long for short string", alloc };
+assert(j.is_string());
+assert(alloc == j.get_allocator());
+
+jsoncons::pmr::json j1{10};
+assert(j1.is_number());
+
+j1 = std::move(j);
+assert(j1.is_string());
+assert(alloc == j1.get_allocator());
+assert(j.is_null());
+```