Add documentation for native_type on tables.

Author: cosmith-nvidia

docs/source/languages/cpp.md

@@ -242,6 +242,40 @@ provide the following functions to aide in the serialization process:
     }
 ```
 
+-   `native_type("type")` (on a table): Tables can also be represented with
+native types. For example, the following schema:
+
+```cpp
+    table Matrix {
+      rows: int32;
+      columns: int32;
+      values: [float];
+    }
+```
+
+Could be represented by a user-defined C++ class:
+
+```cpp
+    class Matrix { ... }
+```
+
+In this case, the following function declarations are generated by the compiler.
+The user must provide and link the matching function definitions:
+
+```cpp
+    struct Matrix FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
+      // ...
+      void UnPackTo(
+          Native::Matrix *_o,
+          const ::flatbuffers::resolver_function_t *_resolver = nullptr) const;
+    }
+
+    ::flatbuffers::Offset<Matrix> CreateMatrix(
+        ::flatbuffers::FlatBufferBuilder &_fbb,
+        const NativeMatrixType *_o,
+        const ::flatbuffers::rehasher_function_t *_rehasher = nullptr);
+```
+
 Finally, the following top-level attributes:
 
 -   `native_include("path")` (at file level): Because the `native_type` attribute