XWIKI-12474: Make it possible to configure available syntaxes at runtime using the Admin UI

* Better doc

xwiki-platform-tools/xwiki-platform-tool-configuration-resources/src/main/resources/xwiki.cfg.vm

@@ -33,7 +33,8 @@
 # xwiki.readonly=no
 
 #-# [Since 1.6RC1] Defines the initial list of supported syntaxes. They list of enabled syntaxes is then controlled by
-#-# the Admin UI of XWiki. When not defined, the default syntax ("xwiki/2.1") is configured by default.
+#-# the Admin UI of XWiki. When not defined, the default syntax is configured by default (this default syntax is
+#-# defined in xwiki.properties using the core.defaultDocumentSyntax property).
 #-# Example of syntaxes:
 #-#    xwiki/2.0, xwiki/2.1, confluence/1.0, jspwiki/1.0, creole/1.0,  mediawiki/1.0, twiki/1.0, xhtml/1.0,
 #-#    html/4.01, plain/1.0, docbook/4.4, markdown/1.0, markdown/1.1, apt/1.0