Enhance model parameter description in models.mdx (#1538)

Clarified the model parameter description to include the format for
specifying both model and provider.

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Co-authored-by: Lauren Hirata Singh <[email protected]>

Author: flingjie

src/oss/langchain/models.mdx

@@ -77,7 +77,7 @@ See @[`initChatModel`][initChatModel] for more detail, including information on
 A chat model takes parameters that can be used to configure its behavior. The full set of supported parameters varies by model and provider, but standard ones include:
 
 <ParamField body="model" type="string" required>
-    The name or identifier of the specific model you want to use with a provider.
+   The name or identifier of the specific model you want to use with a provider. You can also specify both the model and its provider in a single argument using the '{model_provider}:{model}' format, for example, 'openai:o1'.
 </ParamField>
 
 :::python