When a command's 'data' is an object, its doc comment describes the
arguments defined there.  When 'data' names a type, the doc comment
does not describe arguments.  Instead, the doc generator inserts a
pointer to the named type.

An event's doc comment works the same.

We don't actually check doc comments for commands and events.
Instead, QAPISchema._def_command() forwards the doc comment to the
implicit argument type, where it gets checked.  Works because the
check only cares for the implicit argument type's members.

Not only is this needlessly hard to understand, it actually falls
apart in two cases:

* When 'data' is empty, there is nothing to forward to, and the doc
  comment remains unchecked.  Demonstrated by test doc-bad-event-arg.

* When 'data' names a type, we can't forward, as the type has its own
  doc comment.  The command or event's doc comment remains unchecked.
  Demonstrated by test doc-bad-boxed-command-arg.

The forwarding goes back to commit 069fb5b2 "qapi: Prepare for
requiring more complete documentation", put to use in commit
816a57cd "qapi: Fix detection of bogus member documentation".  That
fix was incomplete.

To fix this, make QAPISchemaCommand and QAPISchemaEvent check doc
comments, and drop the forwarding of doc comments to implicit argument
types.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191024110237.30963-12-armbru@redhat.com>