Improve ALTER TABLE documentation

The ALTER TABLE documentation wasn't terribly clear when it came to
which commands could be combined together and what it meant when they
were.

In particular, SET TABLESPACE *can* be combined with other commands,
when it's operating against a single table, but not when multiple tables
are being moved with ALL IN TABLESPACE.  Further, the actions are
applied together but not really in 'parallel', at least today.

Pointed out by: Amit Langote

Improved wording from Tom.

Back-patch to 9.4, where the ALL IN TABLESPACE option was added.

Discussion: https://www.postgresql.org/message-id/14c535b4-13ef-0590-1b98-76af355a0763%40lab.ntt.co.jp

doc/src/sgml/ref/alter_table.sgml

@@ -767,11 +767,11 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
   </para>
 
   <para>
-   All the actions except <literal>RENAME</literal>,
-   <literal>SET TABLESPACE</literal>, <literal>SET SCHEMA</literal>,
+   All the forms of ALTER TABLE that act on a single table, except
+   <literal>RENAME</literal>, <literal>SET SCHEMA</literal>,
    <literal>ATTACH PARTITION</literal>, and
    <literal>DETACH PARTITION</literal> can be combined into
-   a list of multiple alterations to apply in parallel.  For example, it
+   a list of multiple alterations to be applied together.  For example, it
    is possible to add several columns and/or alter the type of several
    columns in a single command.  This is particularly useful with large
    tables, since only one pass over the table need be made.