The gp_ignore_error_table GUC will be deprecated in Greenplum 6 to
be removed in Greenplum 7. Highlight this in the documentation.

Discussion: https://groups.google.com/a/greenplum.org/forum/#!topic/gpdb-dev/mzYcVk_G5UwReviewed-by: NDavid Yozie <dyozie@pivotal.io>