docs: clarify ESDIRK methods do not support SplitODEProblem

Move ESDIRK methods (ESDIRK54I8L2SA, ESDIRK436L2SA2, ESDIRK437L2SA,
ESDIRK547L2SA2, ESDIRK659L2SA) to a separate "Higher-Order ESDIRK"
section and add a note clarifying they do not support SplitODEProblem.

The IMEX SDIRK section now explicitly states that KenCarp methods
support SplitODEProblem for IMEX integration, while ESDIRK methods
do not have this capability.

This addresses confusion where users expected ESDIRK methods to work
with split ODE problems for IMEX integration, but the implementation
treats the entire RHS implicitly.

Fixes SciML#2910

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ChrisRackauckas

docs/src/implicit/SDIRK.md

@@ -80,11 +80,26 @@ Kvaerno5
 
 ### IMEX SDIRK
 
+These methods support `SplitODEProblem` for implicit-explicit (IMEX) integration,
+where the stiff part is treated implicitly and the non-stiff part is treated explicitly.
+
 ```@docs
 KenCarp4
 KenCarp47
 KenCarp5
 KenCarp58
+```
+
+### Higher-Order ESDIRK
+
+Higher-order ESDIRK (Explicit first stage Singly Diagonally Implicit Runge-Kutta) methods.
+These are high-order L-stable implicit methods where the first stage is explicit.
+
+!!! note
+    These methods do not support `SplitODEProblem`. For IMEX integration with split
+    problems, use the KenCarp methods above instead.
+
+```@docs
 ESDIRK54I8L2SA
 ESDIRK436L2SA2
 ESDIRK437L2SA