Add some Doxygen comments for broadphase and linearmath classes.

src/LinearMath/btQuaternion.h

@@ -19,6 +19,7 @@ subject to the following restrictions:
 
 #include "btVector3.h"
 
+///The btQuaternion implements quaternion to perform linear algebra rotations in combination with btMatrix3x3, btVector3 and btTransform.
 class btQuaternion : public btQuadWord {
 public:
 	btQuaternion() {}