Add warning that mutable argument defaults are evaluated only once;

with examples and workaround.  This keeps coming up, and I believe
that this section in the tutorial may have been (in part) the source
of the confusion.  While it didn't show examples with [] for a default,
it also didn't emphasize enough why that would be a bad idea, and
while it did say that defaults are evaluated at the point of function
definition, the example was not relevant for this issue.

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