improvement documentation for input method related features in qtvkb

https://doc.qt.io/qt-6/qtvirtualkeyboard-overview.html#features

Here we have written support for Predictive text, but rather than QVKB support, it relies more on the third-party libraries under it.
For example, in Japanese, we use OpenWnn, and in English, we use Hunspell, but in different platforms, the support of QVKB is different, for example, in Windows, by default, English does not support Predictive text.

So I suggest that instructions should be added to guide the user to configure Hunspell's thesaurus correctly, and if Hunspell has instructions for this, we can add links in our documentation to guide the user.