Choose a default 'safe' collision margin for very small convex collision shapes, in particular btBoxShape and btCylinderShape

and add some documentation in btConvexInternalShape.h
Thanks to Simon Lundmark for the suggestion

Fixes http://code.google.com/p/bullet/issues/detail?id=349

src/BulletCollision/CollisionShapes/btBox2dShape.h

@@ -83,6 +83,7 @@ public:
 	}
 
 
+	///a btBox2dShape is a flat 2D box in the X-Y plane (Z extents are zero)
 	btBox2dShape( const btVector3& boxHalfExtents) 
 		: btPolyhedralConvexShape(),
 		m_centroid(0,0,0)
@@ -97,6 +98,11 @@ public:
 		m_normals[2].setValue(0,1,0);
 		m_normals[3].setValue(-1,0,0);
 
+		btScalar minDimension = boxHalfExtents.getX();
+		if (minDimension>boxHalfExtents.getY())
+			minDimension = boxHalfExtents.getY();
+		setSafeMargin(minDimension);
+
 		m_shapeType = BOX_2D_SHAPE_PROXYTYPE;
 		btVector3 margin(getMargin(),getMargin(),getMargin());
 		m_implicitShapeDimensions = (boxHalfExtents * m_localScaling) - margin;