Expand and document the tokenization search
    
    This basically add support for searching for priority:high in the search
    field at the top of the issues page.
    
    Since it adds the corresponding documentation, it also:
    
    Fixes https://pagure.io/pagure/issue/1908
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>