The behaviour of traitlets.All in observe/unobserve

I'm finding the behaviour of `traits.All` very confusing in `observe` and `unobserve`. Since I can't find much documentation on it, I thought I'd ask here.

My confusion started when I was using observe and unobserve over at ipywidgets. I created jupyter-widgets/ipywidgets#2057 as I thought it was an ipywidgets issue.

That issue is more descriptive (and will clarify this issue for anyone confused), but in short: I expected `.unobserve(names=traitlets.All)` to unobserve _all_ traits for a given function. Instead, I find that I need to specify the same name as I did when I called `.observe()`.

This is of course fine, but then I don't understand the point of `All`, or why it is listed in the docstring as a default argument.

To sum things up: I am confused by the behaviour and purpose of traitlets.All, and by the docstring of observe and unobserve.

To add: I am very impressed by traitlets in general, and I'm really excited to see what more I can use it for - I've already enjoyed using it very much.

Start with the observe and unobserve docstrings and the linked ipywidgets issue to understand the reported behavior of traitlets.All. Document the purpose and behavior of All, including how it applies when observing versus unobserving traits, so the defaults and examples are unambiguous.

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