Add a missing function source argument to the list of arguments for the ...

[lantti]

...rereduce message of the Query Server Protocol

It seems the rereduce message description text had the reduce function source argument missing. The example below was apparently still correct.

[kxepal]

Merged with a bit shorter commit message, thanks!
It's good to have each line under 50/72 characters, but no longer. If you need to add some more notes about the change, the pattern should be the following:

<Short summary for 50/72 chars>

<Long detailed description by 72 chars per line>