Add documentation for writing a custom display for macro parameters in WYSIWYG edit mode

From Marius:

You can follow what the date macro (https://github.com/xwikisas/application-task/tree/master/macro-date) did:

• define a Java type https://github.com/xwikisas/application-task/blob/master/macro-date/macro-date-api/src/main/java/com/xwiki/date/DateType.java
• annotate the macro parameter using the previously defined type https://github.com/xwikisas/application-task/blob/master/macro-date/macro-date-api/src/main/java/com/xwiki/date/macro/DateMacroParameters.java#L52
• write the displayer (for edit) https://github.com/xwikisas/application-task/blob/master/macro-date/macro-date-default/src/main/resources/templates/html_displayer/datetype/edit.vm

From Thomas:

Note that you don't need to use @PropertyDisplayType, you can directly use the type if you write the corresponding converter,

JIRAs for the implemented feature:

• XCOMMONS-1527
• XWIKI-15827