tests/qapi-schema: Convert doc-good.json to rST-style strong/emphasis

tests/qapi-schema: Convert doc-good.json to rST-style strong/emphasis

doc-good.json currently uses the old *strong* and _emphasis_ markup.
As part of the conversion to rST this needs to switch to **strong**
and *emphasis*, because rST uses underscores as part of its markup
of hyperlinks and will otherwise warn about the syntax error.

In commit a660eed482063b we fixed up the in-tree uses of the
old markup:
 1) _this_ was replaced with *this*
 2) the only in-tree use of *this* was left alone (turning
    a 'strong' into an 'emphasis')
(and so currently in-tree nothing is using either new-style
**strong** or old-style _emphasis_).

Update doc-good.json in a similar way:
 1) replace _this_ with *this*
 2) remove the usage of old-style *this*

(This slightly reduces the coverage for the old Texinfo generator,
which is about to go away, but is fine for the new rST generator
because that does not need to handle strong/emphasis itself because
it is simply passing the entire text as raw rST to Sphinx.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-13-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>