Improve wxDC::DrawArc() documentation.

The existing documentation was flat out wrong as it is simply impossible to
pass a circle through any two arbitrary points while also fixing its centre,
explain what this function really does and also use this as an opportunity to
point people to DrawEllipticArc() which is much less confusing.

Also clarify the segments drawing logic of DrawArc() as it is quite
counterintuitive too as it depends on the brush being used and is also
different from DrawEllipticArc() which uses a simpler logic.

See #15609.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75063 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775