Fix documentation for QWebEnginePage::selectionChanged

We should specify in the QWebEnginePage::selectionChanged signal documentation, that it gets emitted for every single visual character change, and not upon selection "end".

• If you use your mouse to select a bit of text, the signal will not be emitted at the end of mouse release, but for each additional character selected.
• If you use shift + arrow keys, signal will be emitted for each additional character selected.