Add documentation for type-traits

This adds documentation for the type-traits.
Remove documentation for non-existent platform methods

This removes the documentation for the non-existent platforms()
and platform_count() methods in the platform class. These methods
have been moved to the system class and are documented there.
Add documentation for platform::get_extension_function_address()

This adds documentation for the get_extension_function_address()
method in the platform class.
Add documentation for platform::unload_compiler()

This adds documentation for the unload_compiler() method in
the platform class.