QDoc: introduce new command to replace \instantiates

The \instantiates command causes some confusion for users, both authors and readers. A lively discussion on a change to the QDoc manual's description of the command made it clear that we can do better.

The new command is proposed to be language agnostic yet descriptive in the relationship it describes; \nativetype

Every QML type has exactly one native type. A native type can have multiple QML types.

This single command should establish this one-to-many relationship.

The output should be of the form "X in Y", as in "Foo in QML / QFoo in C++". This is because the types really are the same on the C++ and QML side, apart from their names.

When the new command is ready, the old \instantiates command shall be deprecated.

Documentation must be updated accordingly.