Log Message:
-----------
Add note about space usage of 'manual' approach to clustering, per
suggestion from Sergey Koposov. Also some other minor editing.
Modified Files:
--------------
pgsql/doc/src/sgml/ref:
cluster.sgml (r1.37 -> r1.38)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/cluster.sgml.diff?r1=1.37&r2=1.38)