Add guidance about disconnecting callbacks

[DragaDoncila]

📚 New content request

We have an events reference that shows how to connect a layer or viewer event, and a guide on hooking up your own events, but none of them mention how to disconnect from that event 😅.

We should document that! Especially because I know in some cases (don't know which, don't know if they're relevant), your connect call returns a function you call to disconnect, whereas in other cases like layer events, you do my_layer.events.blah.disconnect(my_func)

Outline

I think these should be updates to existing docs linked above.

[jni]

I know in some cases (don't know which, don't know if they're relevant), your connect call returns a function you call to disconnect

how sure are you about this? (If so I would expect this is maybe a psygnal thing, but I do not remember anything like this.)

[DragaDoncila]

how sure are you about this?

Not very! I just remember doing this at some stage for napari stuff but you're probably right that it's not any of our public events. I just wanted to flag it so that we actually check

[cnstt]

This issue is linked to #577 and napari/napari#5687 (comment)