qapi: Use rST markup for literal blocks

There are exactly two places in our json doc comments where we
use the markup accepted by the texi doc generator where a '|' in
the first line of a doc comment means the line should be emitted
as a literal block (fixed-width font, whitespace preserved).

Since we use this syntax so rarely, instead of making the rST
generator support it, instead just convert the two uses to
rST-format literal blocks, which are indented and introduced
with '::'.

(The rST generator doesn't complain about the old style syntax,
it just emits it with the '|' and with the whitespace not
preserved, which looks odd, but means we can safely leave this
change until after we've stopped generating texinfo.)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-11-peter.maydell@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qapi/qapi-schema.json

@@ -21,8 +21,10 @@
 #
 # Example:
 #
-# | -> data issued by the Client
-# | <- Server data response
+# ::
+#
+#   -> data issued by the Client
+#   <- Server data response
 #
 # Please, refer to the QMP specification (docs/interop/qmp-spec.txt) for
 # detailed information on the Server command and response formats.