pgsql: Add some real documentation about TOAST (finally).

Log Message:
-----------
Add some real documentation about TOAST (finally). Combine this with
the old 'page' chapter and the recently added 'filelayout' chapter to
make a coherent chapter about PostgreSQL's physical storage layout.

Modified Files:
--------------
pgsql/doc/src/sgml:
diskusage.sgml (r1.13 -> r1.14)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/diskusage.sgml.diff?r1=1.13&r2=1.14)
filelist.sgml (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml.diff?r1=1.40&r2=1.41)
lobj.sgml (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/lobj.sgml.diff?r1=1.35&r2=1.36)
postgres.sgml (r1.72 -> r1.73)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/postgres.sgml.diff?r1=1.72&r2=1.73)
xtypes.sgml (r1.24 -> r1.25)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xtypes.sgml.diff?r1=1.24&r2=1.25)
pgsql/doc/src/sgml/ref:
alter_table.sgml (r1.75 -> r1.76)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_table.sgml.diff?r1=1.75&r2=1.76)

Added Files:
-----------
pgsql/doc/src/sgml:
storage.sgml (r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/storage.sgml?rev=1.4&content-type=text/x-cvsweb-markup)

Removed Files:
-------------
pgsql/doc/src/sgml:
filelayout.sgml
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelayout.sgml)
page.sgml
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/page.sgml)