prefix search in tsearch

[docs from cvs HEAD]

I found the text-search documentation a little unclear about 'prefix search'; specifically, the
examples do not show that the so-called 'prefix' is first stemmed, before it is used as prefix.

For instance, the following can be a little surprising:

SELECT to_tsvector( 'postgraduate' ) @@ to_tsquery( 'postgres:*' );
?column?
----------
t
(1 row)

Because prefix search is such an important functionality I think this should be better explained,
which I hope the attached doc-patch does.

(In textsearch.sgml is another mention + example of prefix search, perhaps it should be extended a
little there too - which I'm happy to do as well, but I first wanted to see if you agree that it
is a little too obscure as it stands)

Erik Rijkers