improve docstring of IMEXEuler

[ranocha]

Xref #1590

[ranocha]

The test failure is unrelated

Linear Nonlinear Solver Tests: Error During Test at /home/runner/.julia/packages/SafeTestsets/A83XK/src/SafeTestsets.jl:25
Got exception outside of a @test
LoadError: MethodError: no method matching ldiv!(::LinearSolve.InvPreconditioner{LinearAlgebra.Diagonal{Float64, Vector{Float64}}}, ::SubArray{Float64, 1, Matrix{Float64}, Tuple{Base.Slice{Base.OneTo{Int64}}, Int64}, true})
Closest candidates are:
ldiv!(::Any, ::ChainRulesCore.AbstractThunk) at /home/runner/.julia/packages/ChainRulesCore/uxrij/src/tangent_types/thunks.jl:88
ldiv!(::Any, ::IterativeSolvers.Identity, ::Any) at /home/runner/.julia/packages/IterativeSolvers/rhYBz/src/common.jl:32
ldiv!(::Any, ::LinearSolve.InvPreconditioner, ::Any) at /home/runner/.julia/packages/LinearSolve/ESgPm/src/preconditioners.jl:29
...

[ranocha]

Looks like buildkite still shows the same error

Got exception outside of a @test

  | LoadError: MethodError: no method matching ldiv!(::LinearSolve.InvPreconditioner{LinearAlgebra.Diagonal{Float64, Vector{Float64}}}, ::SubArray{Float64, 1, Matrix{Float64}, Tuple{Base.Slice{Base.OneTo{Int64}}, Int64}, true})

[ChrisRackauckas]

oh buildkite doesn't reset the cache easily.