pgsql: Docs: clarify treatment of variadic functions with zero variadic

Docs: clarify treatment of variadic functions with zero variadic arguments.

Explain that you have to use "VARIADIC ARRAY[]" to pass an empty array
to a variadic parameter position.  This was already implicit in the text
but it seems better to spell it out.

Per a suggestion from David Johnston, though I didn't use his proposed
wording.  Back-patch to all supported branches.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/7ac0aff2b83e88edf5e7dcd4c4b425319d9537d6

Modified Files
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doc/src/sgml/xfunc.sgml |   14 ++++++++++++++
1 file changed, 14 insertions(+)