Make headers in Advanced Callbacks more generic

[chriddyp]

In https://dash.plotly.com/advanced-callbacks, we discuss PreventUpdate and dash.no_update in the context of "displaying error messages". However, their use is much more broad than that (e.g. https://community.plotly.com/t/sometimes-dont-update-an-output-element-in-a-callback/41984/2)

We should update these headers to be more like "Preventing Updates to Outputs" and then in the example description mention that one use case is to display error messages.