doc: Move ALTER TABLE IF EXISTS description to better place
doc: Move ALTER TABLE IF EXISTS description to better place

It was previously mixed in with the description of ALTER TABLE
subcommands.  Move it to the Parameters section, which is where it is on
other reference pages.

pointed out by Amit Langote

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