Allow other pointer types than unique_ptr for object API.

Change-Id: I945890ce667a2f5a6c0495e78fd5326ed33b9914
Tested: on Linux.
Bug: 30135763

docs/source/CppUsage.md

@@ -99,7 +99,7 @@ construction, access and mutation.
 To use:
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
-    auto monsterobj = GetMonster(buffer)->UnPack();
+    auto monsterobj = UnpackMonster(buffer);
     cout << monsterobj->name;  // This is now a std::string!
     monsterobj->name = "Bob";  // Change the name.
     FlatBufferBuilder fbb;
@@ -125,6 +125,15 @@ same string (or hash).
 When you call `UnPack` (or `Create`), you'll need a function that maps from
 hash to the object (see `resolver_function_t` for details).
 
+# Using different pointer types.
+
+By default the object tree is built out of `std::unique_ptr`, but you can
+influence this either globally (using the `--cpp-ptr-type` argument to
+`flatc`) or per field (using the `cpp_ptr_type` attribute) to by any smart
+pointer type (`my_ptr<T>`), or by specifying `naked` as the type to get `T *`
+pointers. Unlike the smart pointers, naked pointers do not manage memory for
+you, so you'll have to manage their lifecycles manually.
+
 ## Reflection (& Resizing)
 
 There is experimental support for reflection in FlatBuffers, allowing you to