[Doc] Compatibility matrix for mutual exclusive features (#8512)

Signed-off-by: Wallas Santos <wallashss@ibm.com>

vllm/engine/arg_utils.py

@@ -1000,6 +1000,8 @@ class EngineArgs:
             disable_logprobs=self.disable_logprobs_during_spec_decoding,
         )
 
+        # Reminder: Please update docs/source/serving/compatibility_matrix.rst
+        # If the feature combo become valid
         if self.num_scheduler_steps > 1:
             if speculative_config is not None:
                 raise ValueError("Speculative decoding is not supported with "