pgsql: Add GIN documentation.

Log Message:
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Add GIN documentation.

Christopher Kings-Lynne

Modified Files:
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pgsql/doc/src/sgml:
filelist.sgml (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml.diff?r1=1.44&r2=1.45)
xindex.sgml (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xindex.sgml.diff?r1=1.43&r2=1.44)

Added Files:
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pgsql/doc/src/sgml:
gin.sgml (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/gin.sgml?rev=1.1&content-type=text/x-cvsweb-markup)