Document the shift+click selection of mode frequencies in interactive plots

[Nitnelav]

In the interactive plots, the fact that you have to shift+click to select the mode frequencies is a hidden in a Help menu. It could be documented more clearly.

(I found this while reviewing the package for JOSS, but this is not relevant to the submission process so I'm not referencing the JOSS issue intentionally)

[dagghe]

Hi @Nitnelav, I’ve added some instructions in the new example file extra1 and a note in the docs page at the sel_from_plot module. Do you think this is enough or should I add something more, e.g. a pop up windows when the plots are opened?!

[Nitnelav]

Yes anything is fine. Maybe a subtitle of the graph ?