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

greenplum-db/gpdb,techdragon/Postgres-XL,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,ovr/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,ovr/postgres-xl,ovr/postgres-xl,pavanvd/postgres-xl,jmcatamney/gpdb,pavanvd/postgres-xl,techdragon/Postgres-XL,50wu/gpdb,jmcatamney/gpdb,zeroae/postgres-xl,50wu/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,zeroae/postgres-xl,techdragon/Postgres-XL,ashwinstar/gpdb,zeroae/postgres-xl,xinzweb/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,pavanvd/postgres-xl,zeroae/postgres-xl,Postgres-XL/Postgres-XL,xinzweb/gpdb,ashwinstar/gpdb,50wu/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,oberstet/postgres-xl,50wu/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,yazun/postgres-xl,lisakowen/gpdb,yazun/postgres-xl,50wu/gpdb,zeroae/postgres-xl,50wu/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,ovr/postgres-xl,xinzweb/gpdb,adam8157/gpdb,oberstet/postgres-xl,xinzweb/gpdb,yazun/postgres-xl,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,yazun/postgres-xl,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb,oberstet/postgres-xl,adam8157/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,adam8157/gpdb,techdragon/Postgres-XL,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ovr/postgres-xl,50wu/gpdb,oberstet/postgres-xl,lisakowen/gpdb