Document new IRKGaussLegendre arguments

Author: ChrisRackauckas

docs/src/solvers/ode_solve.md

@@ -957,6 +957,23 @@ Pkg.add("IRKGaussLegendre")
 using IRKGaussLegendre
 ```
 
+`IRKGL16(;kwargs...)` has the following arguments:
+
+- second_order_ode (boolean):
+  - =false (default): for a ODEProblem type 
+  - =true: for a second-order differential equation 
+- simd (boolean):
+  - =true: SIMD-vectorized implementation only available for Float32 or Float64 computations
+  - =false (default):  generic implementation that can use with arbitrary Julia-defined number systems 
+- mstep: output saved at every 'mstep' steps. Default 1.
+- initial_extrapolation: initialization method for stages.
+  - =false: simplest initialization
+  - =true (default): extrapolation from the stage values of previous step
+- maxtrials: maximum number of attempts to accept adaptive step size
+- threading
+  - =false (default): sequential execution of the numerical integration
+  - =true: computations using threads (shared memory multi-threading) for stage-wise parallelization
+
 ### SimpleDiffEq.jl
 
 This setup provides access to simplified versions of a few ODE solvers. They