Doc: Correct the red QPushButton style sheet example

The style sheet examples show what happens when a QPushButton gets
a red background and nothing else, and say the result is a flat,
borderless button drawn by the style sheet. That isn't what Qt
does. Without a border, the button keeps its native border and the
platform style is asked to draw it in red. Fusion draws its usual
button in red, styles built from native images get a plain red
button in their place, and some styles ignore the color. Readers
who follow the example get something other than the screenshot.

The example now explains that the result depends on the style and
why, and that specifying a border makes the style sheet draw the
whole button the same way on every platform. The screenshot of the
flat button is gone, since no style produces it.

Pick-to: 6.12
Fixes: QTBUG-49996
Change-Id: Icd79e50af84ea2173ac055d208e281826c733450
Reviewed-by: Topi Reinio <topi.reinio@qt.io>