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/btCylinderShape.cpp

@@ -19,6 +19,8 @@ btCylinderShape::btCylinderShape (const btVector3& halfExtents)
 :btConvexInternalShape(),
 m_upAxis(1)
 {
+	setSafeMargin(halfExtents);
+
 	btVector3 margin(getMargin(),getMargin(),getMargin());
 	m_implicitShapeDimensions = (halfExtents * m_localScaling) - margin;
 	m_shapeType = CYLINDER_SHAPE_PROXYTYPE;