Add partitions command note

Hi,

I am going through the tools documentations for our own ops team. While
doing so, I noticed the following note from the add partition tool:

***Please note that this tool can only be used to add partitions when data
for a topic does not use a key.***

This message left me a bit dubious as to why it would affect my topics or
even the producer. I don't quite follow everything that's happening in
trunk but I saw the tool has been removed there and found the equivalent in
kafka.admin.TopicCommand alterTopic. The message there is a bit clearer and
leaves less place for questioning.

partition " + "logic or ordering of the messages will be affected"

With this, I understand that adding partitions for a topic partitioned with
a key will potentially affect order of messages consumed because they might
end up in different partitions. This is not like adding partitions would
not work :-)

I would gladly update the wiki to clarify this for anyone looking for the
same answer as I was.

cheers,
marc