Document the discrepancy between constructing wxGCDC from
wxGraphicsContext directly or default-constructing it and calling
SetGraphicsContext() later.
Current behaviour is somewhat inconsistent, but useful, because it
allows to draw on wxGraphicsContext using wxDC API using any kind of pen
and brush, even those not supported by this API, while remaining
backwards-compatible (i.e. we can't change SetGraphicsContext() to not
re-apply font, pen and brush, as this would break any existing code
relying on this happening), so it seems like the best alternative.
cc41ddf62a