Refresh documentation for Qt Multimedia on WebAssembly

The documentation for Qt Multimedia is quite focused on the FFmpeg backend, and that native backends have only limited support, see https://doc.qt.io/qt-6/qtmultimedia-index.html. Since we want Qt Multimedia to work well on WebAssembly, we should update the documentation to reflect that WebAssembly is one of the two fully supported backends (WebAssembly and FFmpeg).

Also, the multimedia for WASM page could be freshened up a bit to be more clear about what is supported and what is not supported. Statements such as "Some advanced features may or may not work at this time" should be updated to be more specific. See https://doc.qt.io/qt-6/qtmultimedia-wasm.html.

Platform limitations for individual APIs are documented together with the APIs themselves. Some APIs are documented to only be supported with the FFmpeg media backend, even if they may be supported on WebAssembly as well. We should update the documentation for all new APIs to reflect if they are supported on WebAssembly as well as FFmpeg, if this is the case. We must also review the documentation for other APIs to see if they have any WebAssembly-specific limitations.