mention callback_context in the error message about combining multiple outputs into a single callback

[chriddyp]

dash.exceptions.DuplicateCallbackOutput: 
You have already assigned a callback to the output
with ID "column-graph" and property "figure". An output can only have
a single callback function. Try combining your inputs and
callback functions together into one function.

Users run into this issue for two main reasons:

1. They copied and pasted a previous callback, and now need to update the IDs
2. They want to update an output conditionally based off a certain input changing

For 2, they need to use callback_context and triggered. We should tell them that in this error message.

[alexcjohnson]

re: (2) See also #1054 Trigger inputs

[ToadRider]

I was going to approach this error message but it seems like this may have been already dealt with or made unnecessary by Dash 2.0

If this is still needed I would be happy to update the error message as my team's app uses callback_context heavily.

[LiamConnors]

Closing as the message has been updated and Dash now supports duplicate outputs