doc: Update Q*PaintDevice docs

doc: Update Q*PaintDevice docs

Minor syntax typo in QPagedPaintDevice.

Some Qt 5 updates in QPaintDevice (QGuiApplication)
and a couple of other changes:

1. Usually, we say an object is "painted" or "drawn" when it is
drawn on top of another object (like drawEllipse()) not when it
is the destination of the draw commands. To be more consistent I
updated the language here.

2. What's commonly called a 32-bit display has 24 bits of color
information and eight bits of opacity information. This gives a
color count which can be accurately represented by int. To avoid
confusion, the exception in the docs for colorCount() has been
generalized a little.

Change-Id: Iabaaed1f3e4a80e7e14dfd8855da8c3cd8b3012f
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>