docs: Add callback-related info to virStream{Abort,Finish}

When one has a non-blocking stream and aborts or finishes it without
removing the callback, any event loop invocation will trigger that
callback, but it cannot be removed any more.  We cannot remove the
callback automatically from virStream{Abort,Finish} functions due to
forward-compatibility.  So let's at least document this behaviour,
because it is not easy to find out the reason for.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/libvirt-stream.c

@@ -1131,6 +1131,9 @@ virStreamEventRemoveCallback(virStreamPtr stream)
  * errors, so if this returns a success code the application can
  * be sure that all data has been successfully processed.
  *
+ * If the stream is non-blocking, any callback must be removed
+ * beforehand.
+ *
  * Returns 0 on success, -1 upon error
  */
 int
@@ -1170,6 +1173,9 @@ virStreamFinish(virStreamPtr stream)
  * streams this can be used to inform the driver that it
  * should stop sending data.
  *
+ * If the stream is non-blocking, any callback must be removed
+ * beforehand.
+ *
  * Returns 0 on success, -1 upon error
  */
 int