virCommandFDIsSet: Update documentation

The set of arguments was changed a long time ago (040d9963
which dates back to July 2013)  but the corresponding
documentation was not updated.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NLaine Stump <laine@laine.org>

src/util/vircommand.c

@@ -147,9 +147,8 @@ static int dryRunStatus;
 
 /*
  * virCommandFDIsSet:
- * @fd: FD to test
- * @set: the set
- * @set_size: actual size of @set
+ * @cmd: pointer to virCommand
+ * @fd: file descriptor to query
  *
  * Check if FD is already in @set or not.
  *