Change documentation style and fix minor typos in tests/qtest/libqos/qgraph.h
to automatically generate sphinx documentation in docs/devel/qgraph.rst

The mechanism explanation that once was in qgraph.h is now moved to qgraph.rst

There is no functional change intended.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210308073240.6363-1-eesposit@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>