document api search and sorting

Adds docs for search and sorting - this has been in for a little while, it was just left undocumented because I thought I might tune/change the search indexing a bit. Happy to commit to this though.
well, maybe? the results are ordered by how relevant each result is to the query that was input so while awkward it makes sense.. maybe there's a better way to phrase it, or perhaps just "Defaults to relevance"