Improve more comments in astreamer_gzip.c.

Improve more comments in astreamer_gzip.c.

Duplicate the comment from astreamer_plain_writer_new instead of just
referring to it. Add a further note to mention that there are dangers
if anything else is written to the same FILE. Also add a comment where
we dup() the filehandle, referring to the existing comment in
astreamer_gzip_writer_finalize(), because the dup() looks wrong on
first glance without that comment to clarify.

Per concerns expressed by Tom Lane on pgsql-security, and using
some wording suggested by him.

Discussion: http://postgr.es/m/CA+TgmoYTFAD0YTh4HC1Nuhn0YEyoQi0_CENFgVzAY_YReiSksQ@mail.gmail.com