Update docs

docs/lua_api.txt

@@ -146,6 +146,9 @@ Quaternion
 	**multiply_by_scalar** (a, k) : Quaternion
 		Multiplies the quaternion *a* by the scalar *k*.
 
+	**dot** (a, b) : float
+		Returns the dot product between quaternions *a* a and *b*.
+
 	**length** (q) : float
 		Returns the length of *q*.