KEEPONLYALNUM for pg_trgm is not documented

contrib/pg_trgm in 9.1 becomes more attractive feature by index supports
for LIKE operators, but only alphabet and numeric characters are indexed
by default. But, we can modify KEEPONLYALNUM in the source code to
keep all characters in n-gram words.

However, the limitation and KEEPONLYALNUM are not documented in the page:
http://developer.postgresql.org/pgdocs/postgres/pgtrgm.html

An additonal documentation patches acceptable? The issues would be a FAQ for
non-English users. I heard that pg_trgm will be one of the *killer features*
of 9.1 in Japan, where N-gram based text search is preferred.

--
Itagaki Takahiro