Move the Property Modifier Type documentation

Move the Property Modifier Type documentation

Property Modifier Types are just another sort of QML object type, and
don't deserve explicit top-level documentation.  This commit moves the
docs into the object types documentation, and adds documentation to the
syntax page about the "<PropertyModifierObject> on <propertyName>"
syntax.

Change-Id: Ia9b707739b562d2c8b75fa99b88795ba4d415cf7
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>