Implement (shared) attribute for C++ (#5157)

* Implement native_shared attribute for C++.

Fixes #5141. See also #5145.

* Refine comment in idl.h

* Also refine docs

* Revert "Also refine docs"

This reverts commit 09dab7b45c.

* Also refine docs again.

* grumble

docs/source/CppUsage.md

@@ -88,6 +88,14 @@ convenient accessors for all fields, e.g. `hp()`, `mana()`, etc:
 
 *Note: That we never stored a `mana` value, so it will return the default.*
 
+The following attributes are supported:
+
+-   `shared` (on a field): For string fields, this enables the usage of string
+    pooling (i.e. `CreateSharedString`) as default serialization behavior.
+
+    Specifically, `CreateXxxDirect` functions and `Pack` functions for object
+    based API (see below) will use `CreateSharedString` to create strings.
+
 ## Object based API.  {#flatbuffers_cpp_object_based_api}
 
 FlatBuffers is all about memory efficiency, which is why its base API is written