Update docs for HHVM 4.156

[Andrew]

From hhvm/hhvm.com#199.

Serialize with Options

• "HH\serialize_with_options now supports a disallowObjects option, which disallows serialization of objects except for legacy Hack Collections."

Open question: We never talk about serializing anywhere in the docs. Should we? Or could just start by adding an options section in https://docs.hhvm.com/hack/reference/function/HH.serialize_with_options/ with disallowObjects, though I'm not sure what other options there are.

Literals with Underscores

• Numeric literals can now contain underscores to improve code readability; for example, $x = 1_234_567_890; sets $x to the integer 1234567890."
  Added here: #1197

Breaking: Lambdas in Anonymous functions

• "lambdas contained within an anonymous function will no longer implicitly capture variables from the parent scope of the anonymous function, unless the variable is in the anonymous function's use clause"

Possible areas to update:

• https://docs.hhvm.com/hack/functions/anonymous-functions#legacy-php-style-syntax, and
• https://docs.hhvm.com/hack/expression-trees/syntax-supported#local-variables