doc: Mention SET TABLESPACE clause for ALTER MATERIALIZED VIEW

This command flavor is supported, but there was nothing in the
documentation about it.

Author: Yugo Nagata
Discussion: https://postgr.es/m/20220316133337.5dc9740abfa24c25ec9f67f5@sraoss.co.jp
Backpatch-through: 10

doc/src/sgml/ref/alter_materialized_view.sgml

@@ -43,6 +43,7 @@ ALTER MATERIALIZED VIEW ALL IN TABLESPACE <replaceable class="parameter">name</r
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET COMPRESSION <replaceable class="parameter">compression_method</replaceable>
     CLUSTER ON <replaceable class="parameter">index_name</replaceable>
     SET WITHOUT CLUSTER
+    SET TABLESPACE <replaceable class="parameter">new_tablespace</replaceable>
     SET ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] )
     RESET ( <replaceable class="parameter">storage_parameter</replaceable> [, ... ] )
     OWNER TO { <replaceable class="parameter">new_owner</replaceable> | CURRENT_ROLE | CURRENT_USER | SESSION_USER }