Hello Gavin,
There is a document about SSML in Azure https://video2.skills-academy.com/en-us/azure/cognitive-services/speech-service/speech-synthesis-markup?tabs=csharp
And the original document for SSML as well https://www.w3.org/TR/speech-synthesis/
Hope that helps. Please let us know if you need more details.
Regards,
Yutong