In M, if you adjust the Documentation.FieldDescription and use Value.ReplaceType to apply it, it will be ignored.
Other attributes like Documentation.FieldCaption or Documentation.AllowedValues work fine and will be shown, but the FieldDescription will be ignored.
Please see this function for example: https://gist.github.com/ImkeF/9bd1e4d3a9aecc999fb8883c5fd0a318
The command in row 50 will be ignored.
I have reported this issue to the Product Team: CRI 57093898.
The difference in how Documentation.FieldDescription is processed is due to how we serialize metadata. It is dropped in order to prevent performance issues due to certain sources populating large values for this particular piece of metadata. Even though it appears to be inconsistent with other attributes, this is currently by design. I'm sorry we don't have a fix for you at this time.
If Microsoft is not going to support Documentation.FieldDescription then please change the issue to rejected instead of accepted. Also please change the documentation for Documentation.FieldDescription and indicate it is not supported. I, and others, are writing M language and setting FieldDescription only to later find out it doesnt work and has been an open and accepted issue by Microsoft for 1.5+ years.