Doc update

doc/ref/corelib/json/allocators.md

@@ -14,15 +14,43 @@ A long string, byte string, array or object contains a pointer to underlying sto
 the allocator is used to allocate that storage, and it is retained in that storage.
 For other data members the allocator argument is ignored.
 
-#### Copy construction
+#### Propagation
 
-`basic_json` copy construction 
+The allocator applies not only to a `basic_json`'s data member, but also to its data member's elements. 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);
+
+std::string key1 = "KeyToLongForShortString";
+std::string value1 = "string too long for short string";
+std::string key2 = "AnotherKeyToLongForShortString";
+std::string value2 = "another string too long for short string";
+
+jsoncons::pmr::json j{ jsoncons::json_object_arg, alloc };
+assert(j.get_allocator().resource() == &pool);
+j[key1] = value1;
+j.try_emplace(key2, value2);
+auto it = std::search(std::begin(buffer), std::end(buffer), key1.begin(), key1.end());
+assert(it != std::end(buffer));
+it = std::search(std::begin(buffer), std::end(buffer), value1.begin(), value1.end());
+assert(it != std::end(buffer));
+it = std::search(std::begin(buffer), std::end(buffer), key2.begin(), key2.end());
+assert(it != std::end(buffer));
+it = std::search(std::begin(buffer), std::end(buffer), value2.begin(), value2.end());
+assert(it != std::end(buffer));
+```
+
+#### Copy constructor
+
+`basic_json` copy constructor 
 
 ```
 Json j1(j);
 ```
 
-constructs `j1` from `j`. If `j` is a long string, bytes string, array or object,
+constructs `j1` from `j`. If `j` holds a long string, bytes string, array or object,
 copy construction applies allocator traits `select_on_container_copy_construction` to
 the allocator from `j` (since 0.178.0) For example: 
 
@@ -33,7 +61,7 @@ std::pmr::polymorphic_allocator<char> alloc(&pool);
 
 jsoncons::pmr::json j{ "String too long for short string", alloc };
 assert(j.is_string());
-assert(j.get_allocator() == alloc);
+assert(j.get_allocator().resource() == &pool);
 
 jsoncons::pmr::json j1(j);
 assert(j1 == j);
@@ -42,7 +70,36 @@ assert(j1.get_allocator() == std::allocator_traits<std::pmr::polymorphic_allocat
 assert(j1.get_allocator() == std::pmr::polymorphic_allocator<char>{}); // expected result for pmr allocators
 ```
 
-#### Move construction
+#### Allocator-extended copy constructor
+
+`basic_json` copy constructor 
+
+```
+Json j1(j, alloc);
+```
+
+constructs `j1` from `j`. If `j` holds a long string, bytes string, array or object,
+copy construction uses allocator `alloc` for allocating storage, otherwise `alloc` is ignored. For example: 
+
+```cpp
+char buffer1[1024];
+std::pmr::monotonic_buffer_resource pool1{ std::data(buffer1), std::size(buffer1) };
+std::pmr::polymorphic_allocator<char> alloc1(&pool1);
+
+char buffer2[1024];
+std::pmr::monotonic_buffer_resource pool2{ std::data(buffer2), std::size(buffer2) };
+std::pmr::polymorphic_allocator<char> alloc2(&pool2);
+
+jsoncons::pmr::json j{ "String too long for short string", alloc1 };
+assert(j.is_string());
+assert(j.get_allocator().resource() == &pool1);
+
+jsoncons::pmr::json j1(j, alloc2);
+assert(j1 == j);
+assert(j1.get_allocator().resource() == &pool2);
+```
+
+#### Move constructor
 
 `basic_json` move construction 
 
@@ -54,17 +111,17 @@ initializes `j1` with the contents of `j`, which is either a pointer or a trivia
 and changes the value in `j` to `null`. For example: 
 
 ```
-char buffer[1024] = {};
+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());
+assert(j.get_allocator().resource() == &pool);
 
 jsoncons::pmr::json j1(std::move(j));
 assert(j1.is_string());
-assert(alloc == j1.get_allocator());
+assert(j1.get_allocator().resource() == &pool);
 assert(j.is_null());
 ```
 
@@ -87,13 +144,27 @@ 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());
+assert(j.get_allocator().resource() == &pool);
 
 jsoncons::pmr::json j1{10};
 assert(j1.is_number());
 
 j1 = std::move(j);
 assert(j1.is_string());
-assert(alloc == j1.get_allocator());
+assert(j1.get_allocator().resource() == &pool);
 assert(j.is_number());
 ```
+
+#### Resources
+
+[An allocator-aware variant type](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3153r0.html)
+
+[C++ named requirements: Allocator](https://en.cppreference.com/w/cpp/named_req/Allocator)
+
+[std::allocator_traits](https://en.cppreference.com/w/cpp/memory/allocator_traits)
+
+[std::pmr::polymorphic_allocator](https://en.cppreference.com/w/cpp/memory/polymorphic_allocator)
+
+[std::scoped_allocator_adaptor](https://en.cppreference.com/w/cpp/memory/scoped_allocator_adaptor)
+
+