Added note that SharpDoc is used.

Made it clear that SharpDoc is used to generate API docs, so that people will know where to search when they have questions about how to format documentation, etc.
Added namespace documentation for Microsoft.Xna.Framework.Input.
Minor typo fix in documentation.
Moved namespace documentation into a single file (NamespaceDocs), and included this file in the project definition file.
Re-worded some odd-sounding English.
Added XML documentation for Rectangle.ToString() and the Vector2 constant vectors.
Added XML docs for Vector2.Add()/Subtract().
Cleaned up a few issues in a recent documentation patch.