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

docs/qemu-option-trace.rst.inc

@@ -1,7 +1,7 @@
 
 Specify tracing options.
 
-.. option:: [enable=]PATTERN
+``[enable=]PATTERN``
 
   Immediately enable events matching *PATTERN*
   (either event name or a globbing pattern).  This option is only
@@ -11,7 +11,7 @@ Specify tracing options.
 
   Use :option:`-trace help` to print a list of names of trace points.
 
-.. option:: events=FILE
+``events=FILE``
 
   Immediately enable events listed in *FILE*.
   The file must contain one event name (as listed in the ``trace-events-all``
@@ -19,7 +19,7 @@ Specify tracing options.
   available if QEMU has been compiled with the ``simple``, ``log`` or
   ``ftrace`` tracing backend.
 
-.. option:: file=FILE
+``file=FILE``
 
   Log output traces to *FILE*.
   This option is only available if QEMU has been compiled with