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

doc/src/sgml/ref/create_foreign_table.sgml

@@ -69,8 +69,8 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
    myschema.mytable ...</literal>) then the table is created in the specified
    schema.  Otherwise it is created in the current schema.
    The name of the foreign table must be
-   distinct from the name of any other foreign table, table, sequence, index,
+   distinct from the name of any other relation (table, sequence, index, view,
-   view, or materialized view in the same schema.
+   materialized view, or foreign table) in the same schema.
   </para>
 
   <para>

doc/src/sgml/ref/create_index.sgml

@@ -202,7 +202,10 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
        <para>
         The name of the index to be created.  No schema name can be included
         here; the index is always created in the same schema as its parent
-        table.  If the name is omitted, <productname>PostgreSQL</productname> chooses a
+        table.  The name of the index must be distinct from the name of any
+        other relation (table, sequence, index, view, materialized view, or
+        foreign table) in that schema.
+        If the name is omitted, <productname>PostgreSQL</productname> chooses a
         suitable name based on the parent table's name and the indexed column
         name(s).
        </para>

doc/src/sgml/ref/create_materialized_view.sgml

@@ -77,7 +77,9 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
     <listitem>
      <para>
       The name (optionally schema-qualified) of the materialized view to be
-      created.
+      created.  The name must be distinct from the name of any other relation
+      (table, sequence, index, view, materialized view, or foreign table) in
+      the same schema.
      </para>
     </listitem>
    </varlistentry>

doc/src/sgml/ref/create_sequence.sgml

@@ -46,8 +46,9 @@ CREATE [ TEMPORARY | TEMP ] SEQUENCE [ IF NOT EXISTS ] <replaceable class="param
    specified schema.  Otherwise it is created in the current schema.
    Temporary sequences exist in a special schema, so a schema name cannot be
    given when creating a temporary sequence.
-   The sequence name must be distinct from the name of any other sequence,
+   The sequence name must be distinct from the name of any other relation
-   table, index, view, or foreign table in the same schema.
+   (table, sequence, index, view, materialized view, or foreign table) in
+   the same schema.
   </para>
 
   <para>

doc/src/sgml/ref/create_table.sgml

@@ -124,8 +124,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    schema.  Otherwise it is created in the current schema.  Temporary
    tables exist in a special schema, so a schema name cannot be given
    when creating a temporary table.  The name of the table must be
-   distinct from the name of any other table, sequence, index, view,
+   distinct from the name of any other relation (table, sequence, index, view,
-   or foreign table in the same schema.
+   materialized view, or foreign table) in the same schema.
   </para>
 
   <para>

doc/src/sgml/ref/create_view.sgml

@@ -52,8 +52,8 @@ CREATE [ OR REPLACE ] [ TEMP | TEMPORARY ] [ RECURSIVE ] VIEW <replaceable class
    schema.  Otherwise it is created in the current schema.  Temporary
    views exist in a special schema, so a schema name cannot be given
    when creating a temporary view. The name of the view must be
-   distinct from the name of any other view, table, sequence, index or foreign table
+   distinct from the name of any other relation (table, sequence, index, view,
-   in the same schema.
+   materialized view, or foreign table) in the same schema.
   </para>
  </refsect1>