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.
Merge pull request #2802 from dneelyep/patch-3

Several minor documentation improvements.
Moved namespace documentation into a single file (NamespaceDocs), and included this file in the project definition file.
Merge pull request #2804 from dneelyep/patch-3

Moved namespace docs to a single file.
GraphicsResource.Dispose() requires GraphicsDeviceManager

Fixes issue #2805
Merge pull request #2806 from thefiddler/flushfix

GraphicsResource requires GraphicsDeviceManager
Merge back from develop