Resolution:
Currently there is no documented way to do this but there is however an undocumented feature which allows you to set help information about the function itself.

If you define a public final String[][] called HELP_STRINGS inside your external function class, then this will be inspected for help information about the function. Each String array represents information the function. The first string is the function name and must match exactly (case and all) with a function name in the class. The second string is the help description. The rest of the strings are example result pairs. For example:

public class FunctionTest {
   public static final String[][] HELP_STRINGS = {
      // { &ltfunction name>, &lthelp string>, &ltexample input 1>, &ltexampleoutput 1>, &ltexample input 2>, &ltexample output 2>, ...}
      {"concatTest", "Returns the Concat.", "concatTest(\"Field1\" , \"Field2\")", "Field1 Field2" }
   };

Keywords:- custom java function,xpath editor, description

How can I make my custom Java function to have a description from the XPath editor?