The documentation for ArcGIS Pro 2.7 is currently available, but any specific documentation for ArcGIS Enterprise 10.9 is not yet publicly available.
Keep a lookout for ArcGIS Pro 2.8 and ArcGIS Enterprise 10.9 web help to be available in the near future (estimated mid Q2 of this year).
Once the web help is updated, this page will reflect 2.8 changes
ArcGIS Pro - what's new
Once ArcGIS Enterprise is live, this page will reflect 10.9 changes
What's new - REST API
Utility Network service - REST API