docs(core): add clarity to base token counting methods (#33958)
Wasn't immediately obvious that `get_num_tokens_from_messages` adds
additional prefixes to represent user roles in conversation, which adds
to the overall token count.
```python
from langchain_google_genai import GoogleGenerativeAI
llm = GoogleGenerativeAI(model="gemini-2.5-flash")
num_tokens = llm.get_num_tokens("Hello, world!")
print(f"Number of tokens: {num_tokens}")
# Number of tokens: 4
```
```python
from langchain.messages import HumanMessage
messages = [HumanMessage(content="Hello, world!")]
num_tokens = llm.get_num_tokens_from_messages(messages)
print(f"Number of tokens: {num_tokens}")
# Number of tokens: 6
```
langchain
275dcbf6 - docs(core): add clarity to base token counting methods (#33958)
