Add missing class docstring for PromptAdapterConfig (#17302)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/config.py

@@ -2679,6 +2679,8 @@ class LoRAConfig:
 @config
 @dataclass
 class PromptAdapterConfig:
+    """Configuration for PromptAdapters."""
+
     max_prompt_adapters: int = 1
     """Max number of PromptAdapters in a batch."""
     max_prompt_adapter_token: int = 0