Address Qt 6.7 CMake API review notes for qtdeclarative

See action points from

https://git.qt.io/alcroito/qt6-cmake-api-review/-/blob/master/api_review_qt_6_7.md#qtdeclarative

AP: Double check doc grammar and spelling mistakes

spelling: If .qmlls.ini files already exists -> already exist

AP: Link to variable from qt_add_qml_module docs, already linked from https://doc-snapshots.qt.io/qt6-dev/qtqml-tooling-qmlls.html

AP: Double check if it generates and works with multiple modules in the same project, is it one per module, or one per project

AP: Add introduced in Qt 6.7 blurb

AP: Add example snippet that shows how to pass the arg, e.g. with `-DQT_QML_GENERATE_QMLLS_INI=ON`

AP: Modify doc note to more explicitly mention it's generated in the source dir