For me, the great value of a single page html file for searching the
documentation is that, at least on Firefox, if the page is loaded,
either from a file on the user's computer or from the Internet, to
search for something throughout the entire documentation one only has
to start typing what one is searching for.
The Firefox preference which permits this is "Search for text when I
start typing" which has been carefully hidden in the "Advanced"
preferences under "General".
Once one has typed something that results in a hit, one only has to
press the "Find Next" key combination (Command-G on the Macintosh) to
go through, one by one, all the hits in the document.
Obviously, if you type "the", the number of hits is too great, but one
is generally not searching for "the".
This is, obviously, an automated indexing system that many times
compares favorably with the standard index.