Doxygen docs - async API should state it requires device support

[0xc0170]

Reported here: https://developer.mbed.org/questions/63861/I2CTransfer-Function/?c=21233

We should improve the docs, users should be aware that async methods are not by default available , as for instance other methods (in this case could be sync methods).

[Sissors]

If only there was a way to see which devices support which functions and integrate that on the website ;).

[ciarmcom]

ARM Internal Ref: IOTMORF-221