applies suggestions from Sergey

config/checkdoc/mpich_3.4.3_mpi_waitall.txt → config/checkdoc/mpich_3.4.x_mpi_waitall.txt

-This test checks for a defect found in the 3.4.3 releases of MPICH
-with dataloop.
+This test checks for a defect found in the 3.4-3.4.3 releases of MPICH.
 
 MPI_Waitall causes a segmentation fault if the associated derived datatype and
 communicator are freed before.
+
+The bug is tracked here:
+
+<https://github.com/pmodels/mpich/issues/6595>