Vet/document: send and wait in WebPageTest HAR files get combined into the "wait" event/metric name

[stephendonner]

According to https://github.com/WPO-Foundation/webpagetest/blob/0da83ac3f7e7407c96feaff46af1cfa65c461d6a/www/har/HttpArchiveGenerator.php#L379-L410, send and wait get combined together into wait, with no ability (according to the comment) to separate out those two distinct event timings:

        // represent the difference between the states HAR calls
        // send (time required to send HTTP request to the server)
        // and wait (time spent waiting for a response from the server).
        // We lump both into "wait".  Issue 24* tracks this work.  When
        // it is resolved, read the real values for send and wait
        // instead of using the request's TTFB.
        // *: https://code.google.com/p/webpagetest/issues/detail?id=24