I have created a document library. Next, I wanted to add a Multiple lines of text field and have Append changes only option selected. But, to my surprise, once I choose the field type, I was not provided with the options of Plain or Rich Text formatting, or Append changes only. I am using SharePoint 2010. Were have the options disappeared?
I faced similar problem too, I resolved the issue using site columns, create a custom site columns with the settings you want i.e. append changes option selected multiple line of text, and the site column to your document library, but please do remember to turn on the versioning setting of your doc lib before you add site column