Document array nature of Attribute values

[pp-mo]

We don't really explain that all attribute values are converted to numpy arrays.
This is done by NcAttribute.__init__, and implicitly by NcData/NcVariable.set_attrval.
Meanwhile to fetch, "as_python_value()" is a key feature.

However, direct assignment to NcAttribute.value would bypass it.
It's not clear whether we would want to go further in forcing all attribute values to be arrays.
We could make .value a property, or possibly make NcAttribute itself immutable ??