point doc blocks for builtins to docs.hhvm.com

Summary: ... instead of php.net. This is quite useful for features
 that exist only in HHVM and not PHP5 (such as hack collections).

 Facebook: HipHopSpecific is really quite a misnomer at this point, as
 it seems to be applied only to stuff that isn't documented because
 it's not yet open sourced (and rather rarely internal to the VM).

Reviewed By: @JoelMarcey

Differential Revision: D1693767

Signature: t1:1693767:1416532376:acf87a1bf5dd361a1d1f9cfaa915c32823b973f4
Make BigNode parsable, extend HeapIter to visit BigNodes.

Summary: Now, clients of HeapIter will visit everything in the heap,
not just objects in slabs. Using the standard format isn't
strictly necessary for iterating, but it enables the HeapIter
client to inspect a standard header, to see it's kind.

Reviewed By: @jdelong

Differential Revision: D1687825