Improve QML basic types documentation

Improve QML basic types documentation

Basic types are provided either by the language (int, string, bool,
real, double, date, url, var, variant) or by modules (value types).
In 5.1 we would like modules to be able to provide more basic types,
and thus a cleaner separation of the documentation makes sense.

This patch also improves linking of the term QtQml so that a clear
distinction is made between the QtQml QML module and the QtQml C++
Qt Module (the QtQml QML module links now point to the QtQml type
reference page).

Change-Id: Ibe3ad33e4616c5f29ea38dadc27e13938aedb9d7
Reviewed-by: Bea Lam <bea.lam@nokia.com>