pgsql: Doc: clarify behavior of PQconnectdbParams().

Doc: clarify behavior of PQconnectdbParams().

The documentation omitted the critical tidbit that a keyword-array entry
is simply ignored if its corresponding value-array entry is NULL or an
empty string; it will *not* override any previously-obtained value for
the parameter.  (See conninfo_array_parse().)  I'd supposed that would
force the setting back to default, which is what led me into bug #16746;
but it doesn't.

While here, I couldn't resist the temptation to do some copy-editing,
both in the description of PQconnectdbParams() and in the section
about connection URI syntax.

Discussion: https://postgr.es/m/931505.1606618746@sss.pgh.pa.us

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7fbd1442807ad62ee0e71e2ff3f8b0eefb0af5b8

Modified Files
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doc/src/sgml/libpq.sgml | 101 +++++++++++++++++++++++++++---------------------
1 file changed, 57 insertions(+), 44 deletions(-)