Update guidance docs to reflect grammar support in API (#1775)

# What does this PR do?
Update guidance docs to reflect grammar support in API. The previous
wording was vague and made it sound like openai API supported the
grammar parameter.


https://github.com/huggingface/text-generation-inference/blob/main/router/src/server.rs#L654
confirms that support for grammar is TGI only at this time.

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docs/source/conceptual/guidance.md

@@ -2,7 +2,9 @@
 
 Text Generation Inference (TGI) now supports [JSON and regex grammars](#grammar-and-constraints) and [tools and functions](#tools-and-functions) to help developers guide LLM responses to fit their needs.
 
-These feature are available starting from version `1.4.3`. They are accessible via the [text_generation](https://pypi.org/project/text-generation/) library and is compatible with OpenAI's client libraries. The following guide will walk you through the new features and how to use them!
+These feature are available starting from version `1.4.3`. They are accessible via the [text_generation](https://pypi.org/project/text-generation/) library.  The tool support is compatible with OpenAI's client libraries. The following guide will walk you through the new features and how to use them!
+
+> The Grammar guidance support is currently only available in the TGI API due to lack of support in Open AI API.
 
 ## Quick Start