To make use of the full-text search function in khelpcenter,
you need ht://dig.

ht://dig is a HTML indexing and searching tool.

You can get it from http://www.htdig.org.

NOTE: If you install htdig from source, make sure
that you put the 'htsearch' as well as the 'htdig'
and 'htmerge' binaries into your PATH. 

Per default, 'htsearch' goes to some cgi-bin 
directory.