Document distill for pt-query-digest

One of the most common questions I see about pt-query-digest is "how do I filter the report for queries that use a specific table?"

The answer is something like:

pt-query-digest --filter '$qr->distill($event->{arg}) =~ /\bMyTable\b/i' \
 /var/log/mysql/mysql-slow.log

But use of $qr->distill() is not documented, except a brief mention in the group-by section.

But it is not mentioned that to use the distill result in a filter, we have to call the distill() function because the result of the distill is not a field in the $event object by default.

I suggest this example of filtering by a specific table be added to the pt-query-digest manual, because this use case is pretty popular.