Add plot description to sensors_to_show

[nhoening]

We could add an optional description per plot, which would be displayed when hovering over the plot (see the smart buildings repo / ask Felix) for example vega-lite code.

In the sensors_to_show attribute, the extra attribute would be like this:

[
    {
        "title": "My plot",
        "sensors": [2,3,5],
        "description": "Here we show three sensors, which means you see three lines"
   }
]

We'd adjust the SensorsToShow schema, maybe the patch endpoint for assets, add to the documentation of sensors_to_show, and if editing already has its own place (#1076), then update the form and JS as well.