docs:  clarify use of pg_rewind arguments
docs:  clarify use of pg_rewind arguments

Specifically, --source-pgdata and --source-server.

Discussion: 20160617155108.GC19359@momjian.us

Reviewed-by: Michael Paquier

ashwinstar/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,adam8157/gpdb,ashwinstar/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,xinzweb/gpdb,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb