Better guidance in --result-log deprecation warning

[thebjorn]

The current warnings summary when using --result-log looks like this:

============================== warnings summary ===============================
None
  --result-log is deprecated and scheduled for removal in pytest 4.0
 
-- Docs: http://doc.pytest.org/en/latest/warnings.html
 
==================== 17 passed, 1 warnings in 3.76 seconds ====================

which doesn't give any information on what to use instead, possible work-arounds, links to more information, etc.

The link to warnings.html looks promising but is a red-herring. Perhaps a more distinctive way of indicating that it isn't related to the --result-log warning can be found?

I wasn't actually using the output of --result-log for anything so the easiest solution was just to remove the usage of the switch (but I didn't think to look at the contents until after I'd found #1812 and #830..)

[nicoddemus]

@thebjorn you are absolutely right, we should include a link in the warnings message to the Creating resultlog format files section in the docs, and update that section with guidance for users on how to deal with the deprecation.

I think one of the suggestions was to use pytest-tap or create a new external plugin which would generate a stream of JSON objects.