Add a clarifying note on value_object docs

[aestuans]

The sentence "The JavaScript code does not need to worry about lifetime management" could be misleading on its own.

[aestuans]

@brendandahl just a ping, in case this got lost in your notifications.

[brendandahl]

I'd probably take this a bit further and say it's not recommended to mix the two types of bindings, since value bindings are really meant for making bindings where you don't need to worry about memory management. Something like:

.. note::
    It is not recommended to use fields that correspond to regular C++ binding
    (such as ``class_<T>``) since those properties obey the normal lifetime
    rules of their bound type and may require explicit cleanup in JavaScript.
    See :ref:`Object Ownership <embind-object-ownership>` for more details.