[Misc] Replace TODO in serving transcription (#18895)

Signed-off-by: NickLucche <nlucches@redhat.com>

vllm/entrypoints/openai/serving_transcription.py

@@ -278,7 +278,9 @@ class OpenAIServingTranscription(OpenAIServing):
 
         result_generator: Optional[AsyncGenerator[RequestOutput, None]] = None
         try:
-            # TODO(rob): subtract len of tokenized prompt.
+            # Unlike most decoder-only models, whisper generation length is not
+            # constrained by the size of the input audio, which is mapped to a
+            # fixed-size log-mel-spectogram.
             default_max_tokens = self.model_config.max_model_len
             sampling_params = request.to_sampling_params(
                 default_max_tokens, self.default_sampling_params)