pgsql: Doc: various fixups

Doc: various fixups

* Use <symbol> tags for CONNECTION_* #defines

We were using an inconsistent mix of <literal> and sometimes <function>
tags.

* Use <application> tag for libpq

There was a mix of <literal> and <productname>

Also fix a whitespace issue.

None of these seem critical enough mistakes to backpatch.

Author: Noboru Saito <noborusai@gmail.com>
Discussion: https://postgr.es/m/CAAM3qnJtv5YbjpwDfVOYN2gZ9zGSLFM1UGJgptSXmwfifOZJFQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5e6f9a9c4e7edbe110eab25f9fc05a6d136b950a

Modified Files
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doc/src/sgml/libpq.sgml           | 34 +++++++++++++++++-----------------
doc/src/sgml/logicaldecoding.sgml |  1 -
2 files changed, 17 insertions(+), 18 deletions(-)