pgsql: Add some real documentation about the overall filesystem layout

Log Message:
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Add some real documentation about the overall filesystem layout used by
a Postgres database.  Update page.sgml to match 8.0 tuple header layout.

Modified Files:
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    pgsql/doc/src/sgml:
        filelist.sgml (r1.38 -> r1.39)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml.diff?r1=1.38&r2=1.39)
        page.sgml (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/page.sgml.diff?r1=1.18&r2=1.19)
        postgres.sgml (r1.64 -> r1.65)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/postgres.sgml.diff?r1=1.64&r2=1.65)

Added Files:
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    pgsql/doc/src/sgml:
        filelayout.sgml (r1.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelayout.sgml?rev=1.1&content-type=text/x-cvsweb-markup)