The Form Editor is is very old stuff: It dates back to Smalltalk-80. A description can be found in this book:
Adele Goldberg
Smalltalk-80 The Interactive Programming Environment
Chapter 7
Here is an archive that contains the description of the Form Editor in the form of a HTML document. FormEditor.zip (file size: 54 KBytes) This archive contains a subdirectory. Make sure to keep the subdirectory when you unpack the archive.