XWiki Syntax Guide introduction improvements

There are quite a few leftovers from the introduction of the new Syntax Guide in XWIKI-9176.

Especially the wording of the "introduction" is now somewhat suboptimal and partially just outdated; e.g. as vmassol noted, the first thing people read about XWiki Syntax is that 2.1 is "experimental", which it is no longer for quite some time.