docs(text_splitter): update document of character splitter with tiktoken (#10001)

The current document has not mentioned that splits larger than chunk
size would happen. I update the related document and explain why it
happens and how to solve it.

related issue #1349 #3838 #2140

docs/extras/modules/data_connection/document_transformers/text_splitters/split_by_token.ipynb

@@ -91,7 +91,9 @@
    "id": "de5b6a6e",
    "metadata": {},
    "source": [
-    "We can also load a tiktoken splitter directly"
+    "Note that if we use `CharacterTextSplitter.from_tiktoken_encoder`, text is only split by `CharacterTextSplitter` and `tiktoken` tokenizer is used to merge splits. It means that split can be larger than chunk size measured by `tiktoken` tokenizer. We can use `RecursiveCharacterTextSplitter.from_tiktoken_encoder` to make sure splits are not larger than chunk size of tokens allowed by the language model, where each split will be recursively split if it has a larger size.\n",
+    "\n",
+    "We can also load a tiktoken splitter directly, which ensure each split is smaller than chunk size."
    ]
   },
   {