pgsql: Update discussion of tsearch2 migration.

Log Message:
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Update discussion of tsearch2 migration.  I'm not entirely sure about
the division of material between here and the tsearch2 contrib page,
but at least it's not obviously unfinished any more.

Modified Files:
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    pgsql/doc/src/sgml:
        textsearch.sgml (r1.31 -> r1.32)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/textsearch.sgml?r1=1.31&r2=1.32)