Improve documentation of the display macro

The current documentation of the display macro is explaining less good the main usecase (reusing sections across multiple documents) than the one for the include macro.
However, for this usecase (reusing sections), the main tool is the display macro, but historically it was built after the include macro, so this is probably the reason for the documentation difference.
For example, the include macro documentation clearly says that it allows to include a page in another page, while the display macro does not. Thus, for a beginner, the display macro is more cryptic than the include macro.
See .

Also, the documentation should explicit (with text and screenshots) the "reusing sections" usecase, to target it to regular users of the wiki, not developers (today it's much more developer / advanced user oriented).