VSL manual polish
VSL manual polish

We should avoid the word "received" in the context of response fields,
since we don't "receive" resp.* fields, only beresp. This also fixes
another inconsistency in the manual:

    RespReason - Client response response
        The HTTP response string received.

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache