Document the custom displayer feature and more generally the displayer feature

This was introduced in http://jira.xwiki.org/browse/XWIKI-8085 and documented only in the release notes at: http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWikiEnterprise42M2#HDefaultcustomdisplayersforEasiercustomizationofthewayobjectfieldsaredisplayed

It needs to be moved to reference documentation proper, somewhere to be defined...