Fix X-Text-Size header description

X-Text-Size actually contains the offset to be used for the next start parameter, this header does contain the chunk size itself, and hence the client only needs to pass X-Text-Size header value as the next start.

The value of X-Text-Size header is the return value of writeLogTo method, which is the chunk size + the previous start. https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/model/LargeText.java#L171