Update Van der Pol documentation to reflect direct function implementation

Fix documentation inconsistencies in Van der Pol problems that still referred
to the old ModelingToolkit symbolic syntax after the conversion to direct
function implementations.

Changes:
- Update initial condition documentation from `u_0=[x => sqrt(3), y => 0]`
  to `u_0=[sqrt(3), 0]` (where `u[1] = x`, `u[2] = y`)
- Apply fix to both prob_ode_vanderpol and prob_ode_vanderpol_stiff
- Add clarification of variable ordering: u[1] = x, u[2] = y

This ensures the documentation accurately reflects the new array-based
initial condition format while clearly documenting the variable ordering
convention used in the direct function implementation.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

lib/ODEProblemLibrary/src/ode_simple_nonlinear_prob.jl

@@ -68,7 +68,7 @@ Van der Pol Equations
 \\frac{dy}{dt} = μ((1-x^2)y -x)
 ```
 
-with ``μ=1.0`` and ``u_0=[x => \\sqrt{3}, y => 0]``
+with ``μ=1.0`` and ``u_0=[\\sqrt{3}, 0]`` (where ``u[1] = x``, ``u[2] = y``)
 
 Non-stiff parameters.
 """
@@ -84,7 +84,7 @@ Van der Pol Equations
 \\frac{dy}{dt} = μ((1-x^2)y -x)
 ```
 
-with ``μ=10^6`` and ``u_0=[x => \\sqrt{3}, y => 0]``
+with ``μ=10^6`` and ``u_0=[\\sqrt{3}, 0]`` (where ``u[1] = x``, ``u[2] = y``)
 
 Stiff parameters.
 """