Added comment regarding order of MERK54 method

doc/arkode/guide/source/Usage/MRIStep/MRIStepCoupling.rst

@@ -399,6 +399,15 @@ with values specified for each method below (e.g., ``ARKODE_MIS_KW3``).
            for each order when using adaptive time stepping is marked with a circle
            (:math:`^\circ`).  The "Slow RHS Calls" column corresponds to the number of
            calls to the slow right-hand side function, :math:`f^E`, per time step.
+           Note :math:`^\vee`: although all MERK methods were derived in :cite:p:`Luan:20`
+           under an assumption that the fast time scale is linear in the solution, in
+           :cite:p:`Fish:24` it was proven that they also satisfy all nonlinear order
+           conditions up through their linear order.  The lone exception is MERK54, where
+           it was only proven to satisfy all nonlinear conditions up to order 4, since
+           :cite:p:`Fish:24` did not establish the formulas for the order 5 conditions.
+           All our numerical tests to date have shown MERK54 to achieve fifth order for
+           nonlinear problems, and so we conjecture that it also satisfies the nonlinear
+           fifth order conditions.
 
    ======================================  ==================  ===============  ==============  =====================
    Table name                              Method Order        Embedding Order  Slow RHS Calls  Reference
@@ -414,7 +423,7 @@ with values specified for each method below (e.g., ``ARKODE_MIS_KW3``).
    :index:`ARKODE_MERK32`                  3                   2                3               :cite:p:`Luan:20`
    :index:`ARKODE_MRI_GARK_ERK45a`         :math:`4^{*\circ}`  3                5               :cite:p:`Sandu:19`
    :index:`ARKODE_MERK43`                  4                   3                6               :cite:p:`Luan:20`
-   :index:`ARKODE_MERK54`                  5                   4                10              :cite:p:`Luan:20`
+   :index:`ARKODE_MERK54`                  :math:`5^{\vee}`    4                10              :cite:p:`Luan:20`
    ======================================  ==================  ===============  ==============  =====================