TODO doc for Art namespace.

There are in total 37 xml to document in Art. Right now there are 37 xml not documented in any proportion.
The idea is to have a easy way to know what documentation is missing, if you want to help or are helping please take out the xml that you commit
from the missing list. For documentation that take some time to finish (like HTML, TextBuffer, etc) the best is that you but your name and email
next to the file in this list to acknowledge that you are working in it (is recommendable that you suscribe to the mono-docs  mailing list).

Following is the list of undocumented xml:

Affine.xml
AlphaGamma.xml
AlphaType.xml
Bpath.xml
CompositingMode.xml
DRect.xml
DestroyNotify.xml
FilterLevel.xml
Global.xml
GradientLinear.xml
GradientRadial.xml
GradientSpread.xml
GradientStop.xml
IRect.xml
ImageSource.xml
ImageSourceFlags.xml
MaskSource.xml
PathStrokeCapType.xml
PathStrokeJoinType.xml
Pathcode.xml
PixBuf.xml
PixFormat.xml
Point.xml
Render.xml
RenderCallback.xml
RenderMaskRun.xml
Rgb.xml
Rgba.xml
SVP.xml
SVPRenderAAIter.xml
SVPRenderAAStep.xml
SVPSeg.xml
SvpWriter.xml
Uta.xml
Vpath.xml
VpathDash.xml
WindRule.xml