Explain purpose of 'onInstrumentChangedByEditor' methods (#514)

Left explanatory comments to the methods.

There is a general issue #503 to rework the notification mechanisms.

For now I suggest to close #514 (closed).

This resolves #514 (closed)

added GUI in-code doc labels

requested review from @j.wuttke

assigned to @m.svechnikov

thanks

approved this merge request

merged

mentioned in commit 79014111