JSON documentation needs updating for deprecations

Quoting laknoll from the developer ML:

"The binary JSON support is deprecated and only there for backwards compatibility. It had some issues (e.g. it couldn’t handle large JSON files), that’s why we deprecated it. It’s gone in Qt 6. I guess the docs need some adjustment though."

I believe the following section of the docs is now incorrect:

The 2nd paragraph of https://doc.qt.io/qt-5/json.html says " It also contains support for saving this data in a binary format that is directly "mmap"-able and very fast to access."  This is no longer supported.

thiago FYI