Yelp results in "Official Help Documents" are in presented random order

Spun off from bug #997866.

Results from the Yelp scope are currently randomised before being presented to the user, meaning the results for the same query may end up being in a different order each time the search is run. This was initially done to stop results (especially when no search was entered) from being in the same order each time, with all the results from the same application being grouped together. Without randomising, the results from ubuntu-help would appear at the end, with results from something like boabob's help appearing at the top, which was less than ideal.

Some possible solutions:

 - When no search is done, only present results from ubuntu-help and not from other applications (this still leaves lots of results, though)

 - When no search is done, only present a hardcoded list of results (but which ones?)

 - Find some way to rank yelp results by usefulness