pgsql: Misc documentation fixes.

Misc documentation fixes.

- Misc grammar and punctuation fixes.

- Stylistic cleanup: use spaces between function arguments and JSON fields
  in examples. For example "foo(a,b)" -> "foo(a, b)". Add semicolon after
  last END in a few PL/pgSQL examples that were missing them.

- Make sentence that talked about "..." and ".." operators more clear,
  by avoiding to end the sentence with "..". That makes it look the same
  as "..."

- Fix syntax description for HAVING: HAVING conditions cannot be repeated

Patch by Justin Pryzby, per Yaroslav Schekin's report. Backpatch to all
supported versions, to the extent that the patch applies easily.

Discussion: https://www.postgresql.org/message-id/20201005191922.GE17626%40telsasoft.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/67a6af69870d1c95e6e78295849fee807ba5691b

Modified Files
--------------
doc/src/sgml/catalogs.sgml          |  2 +-
doc/src/sgml/config.sgml            |  4 ++--
doc/src/sgml/dblink.sgml            |  8 ++++----
doc/src/sgml/func.sgml              |  2 +-
doc/src/sgml/gin.sgml               |  4 ++--
doc/src/sgml/high-availability.sgml |  2 +-
doc/src/sgml/isn.sgml               |  2 +-
doc/src/sgml/mvcc.sgml              |  2 +-
doc/src/sgml/parallel.sgml          |  2 +-
doc/src/sgml/plpgsql.sgml           |  8 ++++----
doc/src/sgml/ref/select.sgml        |  2 +-
doc/src/sgml/ref/select_into.sgml   |  2 +-
doc/src/sgml/rules.sgml             |  6 +++---
doc/src/sgml/seg.sgml               |  4 ++--
doc/src/sgml/textsearch.sgml        | 30 +++++++++++++++---------------
15 files changed, 40 insertions(+), 40 deletions(-)