Related Questions:
Is there a place for notes on a Document?
How can I display notes for a Document?
Answer:
Information from the Notes tab is not available as a Merge Field and cannot be pulled into a Document.
Instead we recommend creating a Custom Field for the Booking, such as a Comment Field, which will then be available to use as a Merge Field in your Documents.
Ensure that you enter a Merge Field Name when creating the Custom Field, this is what will allow the field to be used in your Documents.