#21739: mention subtle difference between loops and listcomps in tutorial.

#21739: mention subtle difference between loops and listcomps in tutorial.

We don't want to go into a full explanation of scopes at this point in the
tutorial, so we just mention that the loop creates or overwrites a persistent
variable while the listcomp doesn't.  Not mentioning this would lead someone
to incorrectly assume loops and listcomps were *completely* equivalent, which
would confuse them later.

Original patch by Rose Ames, tweaked to remove the word 'scope'.