In documentation add note about multipart parsing filters

There are a lot of frameworks doing multipart parsing automatically with the help of filters and interceptors. Like org.jboss.seam.web.MultipartFilter in Seam. A multipart request can only be parsed once. So RE users working with multipart should make sure that nothing parses the stream before RE gets it. We should note this in the documentation.

I will do it just adding a reminder to myself.