Comment in Java Jar Manifest
If you are searching for a way to place a comment into the Java Manifest file (MANIFEST.MF) then your search is over. There is no way to place a comment because the specification for the Manifest file has not specified any comment characters.
So no # or / or ; to start a comment.
As others have already posted you can place an key/value pair comment into the Manifest file like
Comment: this is my comment