pgsql: Document filtering dictionaries in textsearch.sgml.

Log Message:
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Document filtering dictionaries in textsearch.sgml.

While at it, copy-edit the description of prefix-match marker support in
synonym dictionaries, and clarify the description of the default unaccent
dictionary a bit more.

Tags:
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REL9_0_STABLE

Modified Files:
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pgsql/doc/src/sgml:
textsearch.sgml (r1.56.2.1 -> r1.56.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/textsearch.sgml?r1=1.56.2.1&r2=1.56.2.2)
unaccent.sgml (r1.3.6.3 -> r1.3.6.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/unaccent.sgml?r1=1.3.6.3&r2=1.3.6.4)