[Task]: (DOC) improve userdefinedjavaclass

### What needs to happen?

Migration: https://issues.apache.org/jira/browse/HOP-4042

https://hop.apache.org/manual/next/pipeline/transforms/userdefinedjavaclass.html is very sketchy.

Questions I have:

- if it's a class, which method(s) is (are) called to do the work?
- how do these methods access the input fields?
- Provide an example of such custom class

Questions on Options:

- "Info transforms": shouldn't this be "Source transforms" or "Input transforms"?
- "Info transforms" and "Target transforms": can both of these be multiple? If so, explain it
- What is "Tag" in relation to the Input and Target transforms?
- "Info transforms" and "Target transforms": the "userdefinedjavaclass" transform is placed on the canvas and connected to some transforms. Then why are these included in the Options?
- "Test class": what exactly happens?

### Issue Priority

Priority: 3

### Issue Component

Component: Documentation

Start with the User Defined Java Class transform manual page at hop.apache.org/manual/next/pipeline/transforms/userdefinedjavaclass.html and review the questions listed in the issue, using migration HOP-4042 for context. Done means explaining the called methods, field access, options and Test class behavior, correcting terminology, and adding an example custom class.

Written by the indexing model from the issue text.