pgsql: Write some real documentation about the index access method API.

Log Message:
-----------
Write some real documentation about the index access method API.

Modified Files:
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pgsql/doc/src/sgml:
catalogs.sgml (r2.95 -> r2.96)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml.diff?r1=2.95&r2=2.96)
filelist.sgml (r1.41 -> r1.42)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml.diff?r1=1.41&r2=1.42)
postgres.sgml (r1.73 -> r1.74)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/postgres.sgml.diff?r1=1.73&r2=1.74)
xindex.sgml (r1.38 -> r1.39)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xindex.sgml.diff?r1=1.38&r2=1.39)

Added Files:
-----------
pgsql/doc/src/sgml:
indexam.sgml (r2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/indexam.sgml?rev=2.1&content-type=text/x-cvsweb-markup)

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