Rewrite docs on user-documentation to use xhp 4 syntax and rules

[lexidor]

How should we tackle this?
Leave the old content available under older versions of the user-documentation (users will need to self-host a copy) or should be document both versions for the time being?

Things of note:

• https://docs.hhvm.com/hack/XHP/some-basics#about-namespaces (:1st_place_medal:)
• https://docs.hhvm.com/hack/XHP/some-basics#runtime-validation
• https://docs.hhvm.com/hack/XHP/basic-usage#creating-a-simple-xhp-object (The :)
• https://docs.hhvm.com/hack/XHP/interfaces#xhproot (renamed to x\node)
• https://docs.hhvm.com/hack/XHP/extending#basics (:x:element and friends)
• https://docs.hhvm.com/hack/XHP/extending#attributes__inheritance (are these helper traits still best practice)
• https://docs.hhvm.com/hack/XHP/extending#children__categories (Category\Phrase)
• https://docs.hhvm.com/hack/XHP/extending#async (Every element is natively async)
• https://docs.hhvm.com/hack/XHP/guidelines#validation-of-attributes-and-children (attributes are no longer validated)

[fredemmott]

Probably keep both in some form until major users have moved; v4 should be considered standard once the typechecker + runtime flags in .travis.sh defaults are flipped

[jjergus]

hhvm/user-documentation#901