Doc: be more precise about conflicts between relation names.

Doc: be more precise about conflicts between relation names.

Use verbiage like "The name of the table must be distinct from the name
of any other relation (table, sequence, index, view, materialized view,
or foreign table) in the same schema." in the reference pages for all
those object types.  The main change here is to mention materialized
views explicitly; although a couple of these pages failed to say
anything at all about name conflicts.

Per suggestion from Daniel Westermann.

Discussion: https://postgr.es/m/ZR0P278MB0920D0946509233459AF0DEFD2889@ZR0P278MB0920.CHEP278.PROD.OUTLOOK.COM