doc: more realistic custom inspect example

Changes the custom inspect example to a more complex object that
actually uses the parameters of the custom inspect function. I
specifically chose a wrapper of a value called a "Box" that inspects
to "Box < inner_inspect_value >".

I also want there to be documentation explaining what the code is
actually doing. E.g., the padding replacement part is to make the
inspected value line up properly when multi-line inputs are given.

I also went with having a space between the Box's brackets and the inner
value because it matches how objects work, and that should definitely be
listed as a convention somewhere in here.

Also, the convention to shorten only when depth is less than 0, not e.g.
at 0.

But I don't know how to write the documentation for that, so I'm leaving
that to somebody who reads this message.

Ref: https://github.com/nodejs/node/issues/8442
PR-URL: https://github.com/nodejs/node/pull/8875


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>