gremlin-server drivers. Documentation change

I added some extra information to the driver section of the gremlin server documentation page. The aim was mostly to clarify some points and give some extra information that otherwise needed to be searched for in javadocs. I think this much would have saved me quite a bit of time while making the PHP driver.

I also changed the image and text for the gremlin-server request bytes as I found the former description was misleading. Correct me if I'm wrong, but gremlin server doesn't expect a "total length" byte as it is handled by the websocket protocol? so I removed that part.

I'm still missing information regarding the `control` opProcessor and the `traverse` operation. So if you have any info or want to fill in the blank in the doc let me know. I just need to know what it is they do to update the documentation.