CompletionResponse docs on fields (run-llama#8120)

* Documented Fields in CompletionResponse

* Documented text and additional_kwargs

llama_index/llms/base.py

@@ -50,7 +50,17 @@ def __str__(self) -> str:
 
 # ===== Generic Model Output - Completion =====
 class CompletionResponse(BaseModel):
-    """Completion response."""
+    """
+    Completion response.
+
+    Fields:
+        text: Text content of the response if not streaming, or if streaming,
+            the current extent of streamed text.
+        additional_kwargs: Additional information on the response(i.e. token
+            counts, function calling information).
+        raw: Optional raw JSON that was parsed to populate text, if relevant.
+        delta: New text that just streamed in (only relevant when streaming).
+    """
 
     text: str
     additional_kwargs: dict = Field(default_factory=dict)