The "modfun" input type is not widely used outside of Riak

Hi, Jason. The "modfun" input type is not widely used outside of Riak,
so its documentation has not spread to most client libraries. A
description of it is available in riak_kv:

https://github.com/basho/riak_kv/blob/master/src/riak_kv_mrc_pipe.erl#L39-60

A module implementing that interface looks like this:

    -module(trend_riak).

    -export([test/3]).

    test(Pipe, _Arg, _Timeout) ->
        Inputs = [{<<"my">>, <<"bu">>}], 这里的inputs可以写成不同的bucket对应相关的key.
        [ ok = riak_pipe:queue_work(Pipe, Input) || Input <- Inputs ], 
        riak_pipe:eoi(Pipe).

And an example usage is:

    riakc_pb_socket:mapred(S,
                           {modfun, trend_riak, test, whatever},
                           [{map, {modfun, riak_kv_mapreduce,
map_object_value}, none, true}]).

Hope that helps,
Bryan