tests/qapi-schema/doc-good: Improve argument description tests

tests/qapi-schema/doc-good: Improve argument description tests

Improve the comments to better describe what they test.

Cover argument description starting on a new line indented.  This
style isn't documented in docs/devel/qapi-code-gen.rst.  qapi-gen.py
accepts it, but messes up indentation: it's stripped from the first
line, not subsequent ones.  The next commit will fix this.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230428105429.1687850-11-armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>