Address Qt 6.8 CMake API review notes for qtwayland

Controls symbol visibility of generated code.

https://codereview.qt-project.org/c/qt/qtwayland/+/544539 - client: Add CMake options to control visibility of generated symbols

https://doc-snapshots.qt.io/qt6-dev/qt-generate-wayland-protocol-client-sources.html

Signature

```cmake
qt_generate_wayland_protocol_client_sources(target
[PUBLIC_CODE | PRIVATE_CODE]
FILES file1.xml [file2.xml ...])
```

Is documented.
NO example snippet.
NO "since qt version" blurb.
Not TP.
Missing link to doc page from cmake manual command reference
https://doc-snapshots.qt.io/qt6-dev/cmake-command-reference.html
Same for the counterpart `qt_generate_wayland_protocol_server_sources`, patch pending.

*Notes:*

*Action points:*
AP: Add since qt version
AP: Add example snippet
AP: Add missing link to doc page
AP: Ask maintainers if it makes sense to introduce a new cmake policy to swap the default to use `PRIVATE_CODE`