get_in function: add example of giving a string to the keys argument

It is currently tempting to test get_in like this:

get_in('x', {'x':5}) # returns 5

and conclude that this will also work:

get_in('test', {'test':5}) # actually returns None

It does not work, because 'test' is treated as ['t','e','s','t']. In complex dictionaries, you may actually get a value, like

get_in('xy', {'x': {'y': 5}} ) # returns 5

The documentation should probably call this out explicitly, if this is the intended behavior, perhaps by giving the 'xy' example above.

I, for one, wouldn't mind an implementation where get_in('test', {'test':5}) returns 5, but I wouldn't go so far as to say that is the right approach. I'm imagining it would facilitate doing something like this:

juxt(*map(curry(get_in), ['str1', ['str2', 'str3'], 'etc']))

Search the documentation for the get_in function and read its current argument description and examples first. Add an explicit example showing that a string keys argument is treated as a sequence of keys, including the nested xy case, and make clear what behavior users should expect. Done means the documentation removes the ambiguity without changing implementation behavior.