Documentation for page_props getter in ParserOutput

I would love to make this getter return null instead of false,
similar to getExtensionData() in the same class. A property that's
not set should be null. Not a boolean. This makes it impossible to
have boolean properties. But I can't change this because it's used
in extensions with "!== false" and such.

Change-Id: I2bc0bed1c58435d791f7a055bac2763a3411bb6f
Merge "Documentation for page_props getter in ParserOutput"