tests/qapi-schema: Demonstrate command and event doc comment bugs

Add negative tests doc-bad-boxed-command-arg and doc-bad-event-arg to
cover boxed and no arguments.  They demonstrate insufficient doc
comment checking.

Update positive test doc-good to cover boxed event arguments.  It
demonstrates the generated doc comment misses arguments.

These bugs will be fixed later in this series.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191024110237.30963-3-armbru@redhat.com>

tests/qapi-schema/doc-bad-boxed-command-arg.out

@@ -0,0 +1,26 @@
+module None
+object q_empty
+enum QType
+    prefix QTYPE
+    member none
+    member qnull
+    member qnum
+    member qstring
+    member qdict
+    member qlist
+    member qbool
+module doc-bad-boxed-command-arg.json
+object Args
+    member a: int optional=False
+command cmd-boxed Args -> None
+    gen=True success_response=True boxed=True oob=False preconfig=False
+doc symbol=Args
+    body=
+
+    arg=a
+an argument
+doc symbol=cmd-boxed
+    body=
+
+    arg=a
+bogus