Update docs and add tests.
[ChangeLog][QtCore] Documented existing support for 'F' format when
converting floating-point numbers to strings in QLocale::toString(),
hence equally for QString's floating-point formatting. Previously it
was supported but the documentation neglected to mention it; it only
differs from 'f' for infinities and NaN.
Change-Id: Ic946c0f7b9e86fdf512daa3124bea57fc664b34b
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>