qemu-option-trace.rst.inc: Don't use option:: markup

qemu-option-trace.rst.inc: Don't use option:: markup

Sphinx 3.2 is pickier than earlier versions about the option:: markup,
and complains about our usage in qemu-option-trace.rst:

../../docs/qemu-option-trace.rst.inc:4:Malformed option description
  '[enable=]PATTERN', should look like "opt", "-opt args", "--opt args",
  "/opt args" or "+opt args"

In this file, we're really trying to document the different parts of
the top-level --trace option, which qemu-nbd.rst and qemu-img.rst
have already introduced with an option:: markup.  So it's not right
to use option:: here anyway.  Switch to a different markup
(definition lists) which gives about the same formatted output.

(Unlike option::, this markup doesn't produce index entries; but
at the moment we don't do anything much with indexes anyway, and
in any case I think it doesn't make much sense to have individual
index entries for the sub-parts of the --trace option.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20201030174700.7204-3-peter.maydell@linaro.org