Add a note about immutability and sendResponses

[joelwurtz]

The sendResponses method return a batch result. User can retrieve response and/or exception with the getResponseFor / getExceptionFor.

If the request is updated in the sendRequests method and this transformed request is used to index the response / exception in the batch result user wil never retrieve the result.

IMO we should add a line for implementation client saying that they must used the original request to index the response / exception.

[dbu]

luckily these problems are obsolete now that we dropped the BatchRequest, right? with async this should work better, right? do we need to do something here or close it?

[sagikazarmark]

Agree, we can close it.