Minor correction in alter_table.sgml

The following sentence in the ALTER TABLE documentation is not entirely
accurate:

"All the actions except RENAME, SET TABLESPACE and SET SCHEMA can be
combined into a list of multiple alterations to apply in parallel."

SET TABLESPACE (in the ALTER TABLE form) can be combined with other
subcommands; for example, following works:

alter table foo set tablespace mytbls, add b int;

Perhaps, it should say something like:

All the actions except RENAME, SET TABLESPACE (when using the ALL IN
TABLESPACE form) and SET SCHEMA can be combined into a list of multiple
alterations to apply in parallel.

Attached is a patch.

Thanks,
Amit