TODO doc for Rsvg namespace.

There are in total 21 xml to document in Rsvg. Right now there are 21 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:

BpathDef.xml
Clone.xml
Css.xml
DefType.xml
DefVal.xml
Defs.xml
Error.xml
Global.xml
GradientStop.xml
GradientStops.xml
Handle.xml
LinearGradient.xml
PSCtx.xml
PaintServer.xml
Pixbuf.xml
RadialGradient.xml
Render.xml
SizeFunc.xml
Start.xml
State.xml
Tool.xml