Doc: improve tutorial section about grouped aggregates.

Doc: improve tutorial section about grouped aggregates.

Commit fede15417 introduced FILTER by jamming it into the existing
example introducing HAVING, which seems pedagogically poor to me;
and it added no information about what the keyword actually does.
Not to mention that the claimed output didn't match the sample
data being used in this running example.

Revert that and instead make an independent example using FILTER.
To help drive home the point that it's a per-aggregate filter,
we need to use two aggregates not just one; for consistency
expand all the examples in this segment to do that.

Also adjust the example using WHERE ... LIKE so that it'd produce
nonempty output with this sample data, and show that output.

Back-patch, as the previous patch was.  (Sadly, v10 is now out
of scope.)

Discussion: https://postgr.es/m/166794307526.652.9073408178177444190@wrigleys.postgresql.org