Improve documentation for var properties

Improve documentation for var properties

Previously, the initialization value assignment semantics of var
properties wasn't explicitly documented.  Since curly braces on the
RHS of an initialization assignment denotes a binding (and curly
braces have a different meaning in JavaScript) confusion could result.

This commit clearly explains how to initialize a var property with an
empty object value.

Task-number: QTBUG-23734
Task-number: QTBUG-23388
Change-Id: Ifa94846b170919a0c893f7dda1421c9fb24bd0db
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>