mprove description that XC does not support any constraint which
needs rows at remote nodes to enforce.

	modified:   doc-xc/src/sgml/ddl.sgmlin
	modified:   doc-xc/src/sgml/filelist.sgmlin
	modified:   doc-xc/src/sgml/indices.sgmlin
	modified:   doc-xc/src/sgml/ref/alter_domain.sgmlin
	modified:   doc-xc/src/sgml/ref/create_domain.sgmlin
	modified:   doc-xc/src/sgml/ref/create_table.sgmlin
	new file:   doc-xc/src/sgml/xc-constraint.sgmlin