Explain the AllGroupImplicit on the UI, wherever the AllGroup is displayed

Currently, when the XWikiAllGroup is set as implicit in xwiki.cfg, the XWikiAllGroup stays empty and on the UI and it's not at all clear, for the administrator of the wiki, that it contains all users that have an account on the wiki.

In order to fix that, either we update the display of the group to display everybody, or we add a message on the group page to explain that even though it appears empty it contains everybody.
Imo, a message is enough as a first step, it's important to mention it somewhere as a first step and then we can polish the solution.

This should, ideally, be handled in 2 places:

• on the page of the group itself, where members are listed - to explain the empty table
• on the listing of the groups in administration, where the number of members are listed and XWikiAllGroup appears with 0 members