Additional doc fixes for configurable TOAST compression.

The grammar changes in commit bbe0a81db6
allow SET COMPRESSION to be used with ALTER MATERIALIZED VIEW as
well as with ALTER TABLE, so update those docs to say that it works.

Also, update the documentation for the pg_column_compression()
to explain that it will return NULL when there's no relevant value.

Patch by me, per concerns from Michael Paquier.

Discussion: http://postgr.es/m/CA+Tgmob9h5u4iNL9KM0drZgkY-JL4oCVW0dWrMqtLPQ1zHkquA@mail.gmail.com

doc/src/sgml/func.sgml

@@ -26256,7 +26256,8 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
        </para>
        <para>
         Shows the compression algorithm that was used to compress a
-        an individual variable-length value.
+        an individual variable-length value. Returns <literal>NULL</literal>
+        if the value is not compressed.
        </para></entry>
       </row>
 

doc/src/sgml/ref/alter_materialized_view.sgml

@@ -40,6 +40,7 @@ ALTER MATERIALIZED VIEW ALL IN TABLESPACE <replaceable class="parameter">name</r
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET ( <replaceable class="parameter">attribute_option</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] )
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> RESET ( <replaceable class="parameter">attribute_option</replaceable> [, ... ] )
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
+    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 ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] )