Improve parameter description for the macro on exo

Idea: improve https://extensions.xwiki.org/xwiki/bin/view/Extension/Kroki%20Diagram%20Rendering%20Macro/#HParametersDefinition to indicate what's optional/mandatory as do for other macros.

Here's an example: https://extensions.xwiki.org/xwiki/bin/view/Extension/Include%20Macro#HParametersdefinition