pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/76dd015e859845fc5fe60557328e3a6bbf11be31

Modified Files
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src/fe_utils/astreamer_gzip.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)